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Frequently Asked Questions

The page contains a list of frequently asked questions related to USGS streamgages and other monitoring data. If you'd like more information, have suggestions for new questions, or discover outdated links, please contact us at the Webmaster link at the bottom of the web page.

    (A) USGS streamgaging nationally


  1. Why does the USGS collect streamflow information?
          What is USGS streamflow information used for?
  2. What is the history of USGS streamgaging?

    (B) USGS streamgaging in Minnesota


  1. What are the oldest active streamgages in Minnesota?
  2. Where are streamgages in Minnesota and why are they located there?
  3. How do streamgages operated by the USGS differ from other agencies?
  4. How are streamgages funded?
  5. Why do several agencies seem to have a streamgage at the same location, for instance the USGS, National Weather Service, U.S. Army Corps of Engineers, Minnesota Department of Natural Resources?
  6. Why is streamgage data on the USGS web page sometimes different than that on the National Weather Service, U.S. Army Corps of Engineers, or MN Department of Natural Resources web page?
  7. Do agencies collaborate on stream monitoring efforts?
  8. Why are streamgages discontinued?

    (C) How streamgages work


  1. How does a streamgage work?
  2. How is streamflow or discharge determined?
  3. What types of streamgages are there?
          Why do some stations not have discharge?
  4. What is the "datum" of a gage?
  5. How do I convert the stage or gage-height to an elevation above mean sea level?
  6. Why are there different vertical datums used at gages and what are they?
  7. How does the stage or gage-height relate to the depth of a stream?
  8. How does data get from the streamgage to the internet?
          How often is data updated?
  9. How is the accuracy of streamgage data assured?

    (D) Data retrievals and tips, statistics, and other information and analyses from streamgages


  1. Where can I retrieve data for streamflow/water-quality/suspended sediment/groundwater?
  2. Where can I find all of the types and periods of data collected at a monitoring site?
  3. Have there ever been any streamgages in my river basin?
  4. How can I retrieve data for discontinued streamgages?
  5. What time increments of streamflow data are available (for example: annual, daily, or hourly) and where can I get them?
  6. How can I retrieve data for many sites at once?
  7. Why can't I retrieve rain gage or precipitation data that is more than 120-days old?
  8. Why can't I retrieve long-term stage or gage height for streamgages?
  9. What is the difference between provisional and approved data?
  10. How do I assess the accuracy of provisional streamgage data?
  11. How do I import data into a spreadsheet?
  12. What are some data retrieving tips and shortcuts?

  13. Where can I retrieve streamflow statistics for daily-discharge, peak-flow, high-flow, floods, droughts, or low-flow?
  14. Where can I find additional, value-added, information and analyses from this streamgage?
  15. Where can I find information on streamflow trends?

  16. Why is discharge sometimes not displayed on a streamgage web page?
  17. What is the speed or velocity of the stream I'm interested in?

    (E) Flood-related, drought-related, and current streamflow conditions in context


  1. We've had several "100-year" floods. How can that be?
  2. Where can I view previous USGS publications about floods in Minnesota?
  3. Where can I retrieve streamflow statistics for peak-flow, high-flow, and floods?
  4. How do I convert the stage or gage-height to an elevation above mean sea level?
  5. How does the stage or gage-height relate to the depth of a stream?
  6. How can I receive alerts from a streamgage automatically? What is WaterAlert?

  7. Where can I find information on current or past drought conditions?
  8. Where can I retrieve streamflow statistics for low-flow conditions and droughts?

  9. Is this stream unusually low or high for this time of year?

    (F) Information about specific rivers, streams, and gages


  1. How did the Red River of the North get it's name?
  2. Why does the Red River of the North flow north?
  3. Why does the Red River of the North seem to flood so easily?
  4. What is the history of the streamgage at the Mississippi River at St Paul?

    (G) Miscellaneous questions


  1. What is the USGS National Streamflow Information Program?
  2. What are USGS Cooperative Matching Funds?
  3. How can I receive alerts from a streamgage automatically? What is WaterAlert?
  4. How can I quickly find out the stage and discharge for a river I am on? What is WaterNow?
  5. What is the difference between streamflow and discharge?
  6. What is the difference between a lake and a reservoir?
  7. Where can I find a glossary of water terms?

(A) USGS streamgaging nationally

Q: Why does the USGS collect streamflow information?
         What is USGS streamflow information used for?

A: The USGS is a non-regulatory agency that collects accurate and unbiased streamflow information to serve many different Federal, State, tribal, local, and public needs. Common uses include:

While a given streamgage originally may have been established for one specific purpose, with time it often serves other needs as well, as other agencies, water users, and the public become aware of and depend on the data. The more uses of data or data users that a streamgage acquires, the more valuable that streamgage becomes to the public. The USGS tries to coordinate installation of streamgages with known data users to meet as many needs as possible. See also FAQ B-2 "Where are streamgages in Minnesota and why are they located there?".

More information about streamgage uses is available from the USGS Office of Surface Water , at this Fact sheet about recreation, boating and fishing.

A summary was presented in a 1998 report to Congress.

Return to FAQ List.

Q: What is the history of USGS streamgaging?

A: The first streamgage operated by the U.S. Geological Survey (USGS) was established in 1889 on the Rio Grande near Embudo, New Mexico. Specific appropriation for streamgaging was made by the Organic Act of August 18, 1894, which contained the budget line item of $12,500 "for gauging the streams and determining the water supply of the United States, including the investigation of underground currents and artesian wells in the arid and semiarid sections" (Grover and Hoyt, 1906).

In Minnesota, Federal streamgages have been in operation since the late 1800s. The Federal agencies that have operated them have changed through time but the USGS has published and archives the streamflow records from these streamgages. The longest continuous-record streamgages in Minnesota are listed in FAQ B1.

More information about the history and establishment of the USGS is available in USGS Circular 1050 and USGS Fact Sheet 2009-3048 from Pennsylvania Water Science Center. The authorization of the USGS to conduct water resources investigations is summarized in this memorandum .

Return to FAQ List.

(B) USGS streamgaging in Minnesota

Q: What are the oldest active streamgages in Minnesota?

A: The Mississippi River at Grand Rapids has the longest continuous record of daily discharge in the state, beginning September 17, 1883. The second longest record is from the Mississippi River at St Paul, which began March 1, 1892. More information about the history of the streamgage at St Paul is noted in USGS Scientific Investigations Report 2014-5079. Additional active streamgages with continuous daily discharge records that began more than 100 years ago include the following. Note that some may have gaps in their records.

*Streamgages with long-term records of streamflow that is predominantly unregulated. Unregulated streamflow records are important for determining trends, such as those caused by changes in climate, land use, or other factors. For more information on streamflow trends see the FAQ D-15 "Where can I find information on streamflow trends?".

Return to FAQ List.

Q: Where are streamgages in Minnesota and why are they located there?

A: Interactive maps of USGS streamgages in Minnesota are available from NWISWeb and NWIS Mapper.

Streamgages typically are located where they are most needed for one or more of the many uses of streamgage data ( FAQ A-1 ). Determining a specific streamgage location requires balancing the needs of the data users with the site characteristics needed to ensure the most accurate and robust streamflow record.

For example, a community (data user) needing a flood-forecast streamgage may wish to locate it upstream of the community in order to give advanced warning, or directly in the community near infrastructure to be protected during a flood. The USGS would try to locate the streamgage as near as possible to the needs of the data user(s), but may propose locating it in a reach that would enable the collection of more accurate streamflow data, or is better protected from flooding, etc; or may coordinate with other data users to ensure the location best fits the needs of all. Once a streamgage is established, it is generally not moved without careful coordination among data users and consideration of how the move may affect the previous record of the streamgage. One recent example of re-locating a streamgage at the Mississippi River at St Paul, MN and the considerations that are affected are briefly described in USGS Scientific Investigations Report 2014-5079.

Return to FAQ List.

Q: How do streamgages operated by the USGS differ from those operated by other agencies?

A: USGS streamgages are operated using nationally consistent methods, instrumentation with stringent accuracy requirements, and quality-assurance practices so that streamgage information are comparable across the nation and through time. Data consistency and comparability are essential for studies that compare streamflow characteristics or evaluate trends. Other agencies may operate streamgages for different objectives or time scales, and consequently may not use or require the same methods, instrumentation, and quality assurance practices as the USGS. Data users should use caution when comparing streamflow data among different agencies and check with originating agencies concerning their data collection methods and quality assurance practices.

Methods used by the USGS are documented in techniques and methods reports. The USGS also published a report that qualitatively compares the costs and streamflow information products among USGS and three non-Federal agencies that used similar methods.

Return to FAQ List.

Q: How are streamgages funded?

A: Streamgages operated by the USGS are funded by the USGS and frequently also in cooperation and with funding support from other Federal, State, regional and local governments, and hydroelectric power utilities. USGS funding generally comes from one of two sources, the Cooperative Matching Funds (formerly the Cooperative Water Program) or the National Streamflow Information Program .

In Minnesota, the USGS and more than 40 other agencies and hydroelectric power facilities fund streamgage operations. Some agencies contribute funding to several streamgages state-wide, while others contribute funding to a specific streamgage. Agencies that contribute funding to each applicable streamgage are posted at the top of each streamgage web page on NWISWeb.

More information about the National USGS streamgage network is available in the most recent report to Congress .
For more information, contact us at the link below.

Return to FAQ List.

Q: Why do several agencies seem to have a streamgage at the same location, for instance the USGS, National Weather Service, U.S. Army Corps of Engineers, or Minnesota Department of Natural Resources?

A: Streamflow data from a streamgage may exist on several agencies' web pages. For example, streamflow data from the USGS streamgage at the Crow River at Rockford is available from web pages maintained by the


However, only the USGS is responsible for visiting, operating, and maintaining this streamgage, and for computing the streamflow. Other agencies may download the telemetered data transmissions directly or replicate USGS data on their computer systems for their operations. Data users should refer questions about specific streamgages to the agency that operates the gage.

All streamgages listed on USGS NWISWeb in Minnesota are operated and maintained by the USGS. If you have questions about the accuracy or status of a gage, please contact the USGS at the link at the bottom of the streamgage web page.

Return to FAQ List.

Q: Why is streamgage data on the USGS web page sometimes different than that on the National Weather Service, U.S. Army Corps of Engineers, or MN Department of Natural Resources web page?


As described in the previous FAQ, data from streamgages operated by the USGS are shared with other agencies in Minnesota, including the National Weather Service, U.S. Army Corps of Engineers, and MN Department of Natural Resources. These agencies obtain the data from different points and times in the data stream: some directly from the streamgage each hour, and some from the USGS data base daily. As operators of the streamgage, USGS staff may make corrections to data that are not available at the streamgage or not immediately available to other agencies. In these cases data shown on each agencies web page may be temporarily different.

In Minnesota, USGS only displays data on our NWISWeb pages for streamgages operated by the USGS. For these streamgages, the USGS is the primary agency responsible for visiting, operating, and maintaining streamgage equipment and for maintaining an accurate streamflow record. Users should consider USGS streamgage record the primary record for gages shown on both USGS and other agency web pages. If users find data discrepancies among each agency, the user should refer to the other agencies because their data base may not contain all the corrections made by the USGS.

Q: Do agencies collaborate on stream monitoring efforts?


In Minnesota, Federal and State agencies collaborate closely on streamgaging efforts to minimize duplication of efforts. As described in FAQs B5 and B6 , several agencies may appear to operate streamgages in the same location, but in reality each agency is sharing one data stream from the same gage and using these data for different agency missions and systems. For example, the USGS is a non-regulatory agency responsible for maintaining and accurate and unbiased record of streamflow, the U.S. Army Corps of Engineers uses streamflow in their real-time operations for water control, flood fighting, and river transportation, the National Weather Service uses streamflow in their real-time operations for flood and drought forecasting, and the MN Department of Natural Resources in their operations for water permit regulations and recreation.

In the field, other agencies may be observed visiting a streamgage maintained by the USGS or vice versa. Generally, these agencies are visiting the site for other purposes and with other equipment such as sampling for water, fish, or other aquatic biota, These other efforts are frequently performed at locations with streamgages because it is essential to know the context of the streamflow conditions at the time of sample collection, or because a record of streamflow is needed to compute the loads of chemicals and constituents carried in the stream.

Return to FAQ List.

Q: Why are streamgages discontinued?

A: Streamgages are discontinued when the original purpose(s) of collecting streamflow data has ended or when funding is cut by one or more agencies supporting the streamgage.

This may cause unintended consequences if other agencies or data users began to rely on the data from the streamgage without knowledge of it's funding status, or without the USGS knowing about the newer data users. Because of this, the USGS makes every attempt to alert data users of an impending streamgage cut by posting messages on the streamgage's NWISWeb page, by updating a National Map of Threatened USGS Streamgages , by notifying emergency and water-resource agencies about pending cuts, and by searching for alternative funding partners. For questions about specific gages, contact the USGS at the link on this web page..

Return to FAQ List.

(C) How streamgages work

Q: How does a streamgage work?

A: Streamgages commonly are installed to obtain two types of information about the stream: the water level (or stage) and the streamflow (or discharge) rate. Currently, no instrumentation exists to directly measure the discharge rate that is economical and meets required accuracy needs.

Consequently, most streamgages work by sensing and recording the water level (or stage) through various types of instruments at regular intervals (generally every 15-minutes in Minnesota). Because the streamflow (or discharge) rate cannot be sensed directly in this way, field staff make frequent visits to the site in order to make simultaneous measurements streamflow and the stage directly. From paired observations of stage and discharge, staff develop and maintain a mathematical relation between the stage and discharge, so that discharge can be computed for any given stage.

Fact sheet 2011-3001 provides a good description of how streamgages work and the process of computing streamflow and delivering it to the internet. Other good general references include Fact sheet 2007-3043 and Fact sheet 2005-3131 .

Return to FAQ List.

Q: How is streamflow or discharge determined?

A: At most streamgages, the water level or stage of a stream is measured and recorded continuously, and discharge is measured periodically over the range in stages that occur. From the paired, simultaneous measurements of stage and discharge, a stage-discharge relation is developed that allows discharge to be computed for any stage.

This Fact sheet 2011-3001 provides a good description of the process of computing streamflow and delivering it to the internet. Other good general references include Fact sheet 2007-3043 and Fact sheet 2005-3131 .

Return to FAQ List.

Q: What types of streamgages are there?
      Why do some stations not have discharge?

A: The USGS uses the term "streamgage" to refer specifically to a gaging station used to collect a continuous (every 15-minutes, hourly, and/or daily) record of stage, which is used to compute a continuous record of discharge (streamflow). These are the most common type of gages listed on the USGS NWISWeb streamflow web page. Informally, the term streamgage may also refer to other types of gaging stations that are used for different purposes.

A second type of continuous recording station may record only the stage (also known as water-level or gage-height) of a stream, lake, or reservoir. No discharge or streamflow is measured or computed for these types of gaging stations. These less costly gages are often used to observe trends in water levels, when discharge is not needed or not applicable (lakes). Although not referred to as "stage", water levels from groundwater wells are collected similarly.

A third type of station is known as a crest-stage gage or partial record station. The USGS in Minnesota operates a network of more than 70 crest-stage gages in cooperation with the Minnesota Department of Transportation. These non-continuous gages record only the peak stage of the greatest runoff event that has occurred since the last gage inspection. Peak discharge is computed from these recordings to determine the annual peak stage and discharge. Crest-stage gages are used to develop annual flood-frequency statistics that aid in determining the designs of culverts, bridges, and roads. Information from crest-stage gages also is useful in determining trends in annual peak streamflow.

Return to FAQ List.

Q: What is the "datum" of a gage?

A: The USGS most commonly reports water-level (stage or gage-height) recorded by a streamgage in terms of a "local gage datum." Local gage datum is an arbitrary datum established when the streamgage is installed. Using a local gage datum that is independent of a national vertical datum eliminates confusion over the various and occasionally revised national datums.

The local gage datum generally is related or tied to an elevation above mean sea level by surveying between a reference point established in the local gage datum network and a monument in the National Geodetic Survey network to determine the elevation above mean sea-level. Several national vertical datum networks have been established in the United States through time. Each generally provides slightly different values of elevation above mean sea level (MSL) at a given point. The most recent is the North American Vertical Datum of 1988 (NAVD 88). this is the network that the USGS is tying gages to as resources allow. Older datums also exist, including the National Geodetic Datum of 1929 (NGVD 29), among others. Because elevations are not consistent between National datum models, users should use caution when comparing. More information on USGS use of National geodetic surveys are summarized in USGS Techniques and Methods 11-D1 .

Return to FAQ List.

Q: How do I convert the stage or gage-height from a streamgage to an elevation above mean sea level?

A: As explained in the previous FAQ C-4 , the stage of a streamgage generally refers to a local gage datum, but can be converted to an approximate elevation above mean sea level (MSL), as shown in the example below. Note that although stage from USGS streamgages is generally reported to an accuracy of 0.01 ft, the resulting conversion to elevation above MSL can be much less accurate, because it depends on the accuracies of the methods and information available to establish the elevation above MSL at the gage.

To convert the stage to an approximate elevation above sea level, add the stage reading to the elevation of the gage above MSL to which the gage was surveyed. The MSL adjustment to apply to a given streamgage is listed on the streamgage's NWISWeb page. On the NWISWeb page showing data for a particular streamgage, use the pull-down options under "Available data for this site" and select "SUMMARY OF ALL AVAILABLE DATA", then under DESCRIPTION, note the "Datum of gage" value and national geodetic datum used.

For example, a streamgage has a stage reading of 12.31 ft (local gage datum) on NWISWeb. The streamgage "SUMMARY OF ALL AVAILABLE DATA" indicates the datum of the gage is: 893.08 feet above MSL (NAVD-88). Then the approximate elevation of the water level above MSL is:

       12.31 (stage reading of gage in local gage datum)
    + 903.08 (NAVD-88)
    = 915.39 ft (approximate elevation of water surface above MSL, NAVD-88)

Return to FAQ List.

Q: Why are there different vertical datums used at gages and what are they?

A: Various vertical datums have been established for parts or most of the United States over the years. Not all streamgages are tied to the same datum. There are several national geodetic datum models of the U.S. The two most commonly used in Minnesota are:

The NAVD-88 is more recent and improved, but many streamgages established prior to 1988 use the older datum. A few streamgages in Minnesota may use even older or special datums, including a 1912 datum or a Lake of the Woods datum. USGS is converting gage datum to NAVD-88 where feasible and as resources allow.

Because of the way the datum models vary, differences between the two datums vary by location. Many sites are set to an arbitary datum (gage datum) but not tied to a 'sea level' datum. The U.S. Army Corps of Engineers has information regarding different datums on this web page .

More information about vertical datum and USGS usage is available in this USGS Techniques and Methods Report .

Return to FAQ List.

Q: How does the stage or gage-height relate to the depth of a stream?

A: The stage of a stream generally does not indicate the depth of the stream, the bottom of the channel, or zero-flow conditions. One may calculate an approximate depth of the stream specific to the location in the stream that a discharge measurement was made by subtracting the maximum depth observed during the discharge measurement from the stage reading when the measurement was made. Records of these discharge measurements are on file at the USGS. However, stream channels continually scour and fill, and depths vary by location along the stream, so depths estimated in this way may not reflect current conditions.

When a streamgage is installed, a local gage datum (see related FAQ C-4 ) is established with a reference value that reasonably ensures that the stage of the stream will not fall below a stage of 0.00 ft, even if the stream scours or erodes in the future. For example, a streamgage may be established on a stream adjacent to a bridge. From a known reference point on the bridge, the vertical distance to the water is found to be 16.20 ft. The water is found to be 3.50 ft deep below this point on the bridge. The reference point on the bridge now needs to be assigned an arbitrary elevation so that future readings of the water surface made from the same point are comparable. If the point on the bridge is given an arbitrary elevation of 20.00 ft, then the stage of the stream would be 20.00 ft - 16.20 ft = 3.80 ft. Since the stream was found to be 3.50 ft deep, we could estimate that if the water level receded to the point that the stream was almost dry at this location, that stage of the stream would be: 3.80 ft - 3.50 ft = 0.30 ft. However, it's easily possible that the stream channel could scour 0.5 feet deeper at that location during a flood event, which would could then result in negative stage values (-0.20 ft). Negative stage readings make for confusing computation and reporting. So instead, the arbitrary elevation of the point on the bridge might be assigned a value of 30.00 ft so that future stage readings would not be expected to be less than 0.00 ft

Return to FAQ List.

Q: How does data get from the streamgage to the internet?
      How often is data updated?

A: In Minnesota most USGS streamgage data is recorded by the streamgage every 15 minutes and then transmitted every hour via satellite radio to the USGS. This Fact sheet 2011-3001 provides a good description of the process of computing streamflow and delivering it to the internet. Other good general references include Fact sheet 2007-3043 and Fact sheet 2005-3131 .

Return to FAQ List.

Q: How is the accuracy of streamgage data assured?

A: The USGS invests significant resources to ensure that streamflow data are accurate; from independently testing instrumentation specifications that are provided by vendors, to developing, publishing and adopting standard techniques and methods reports for consistent data collection, to regular internal review of the methods and procedures used by each USGS office. While the quality assurance practices involved in streamgaging are too numerous to list here, one general reference is USGS Open-File Report 94-382 ; the template for a document that each USGS office is required to complete and regularly update if collecting surface-water data.

Return to FAQ List.

(D) Data retrievals, statistics, and other analyses from streamgages

Q: Where can I retrieve data for streamflow/water-quality/suspended sediment/groundwater?

A: Most USGS water-monitoring data for Minnesota, including streamflow, groundwater levels, and water-quality are available on line from NWISWeb pages. To search for a specific river or location, use the NWISWeb Mapper interface or enter the search terms of interest (for example stream name or county) by following the links on the NWISWeb site information page . USGS reports may contain investigation-specific data as well and are available on line at the USGS Publications Warehouse by entering applicable search terms. A general resource that contains links to other types of USGS water-related data nationwide is available Water Data Discovery page.

Return to FAQ List.

Q: Where can I find all of the types and periods of data collected at a monitoring site?

A: To view an inventory of the data collected at a USGS monitoring site,

at the site's NWISWeb page, under the banner "Available data for this site,"
select "SUMMARY OF ALL AVAILABLE DATA" from the drop-down list and
click "GO."

This will generate a table of the types of data collected, period of collection, and counts of the data points for each type of data. For example, here is a link to an inventory of data from the Minnesota River near Jordan. Other agencies may also collect data at USGS monitoing sites, but these data are not maintained by the USGS. See FAQ B-6 for more information.

Return to FAQ List.

Q: Have there ever been any streamgages in my river basin?

A: To search for a specific river or location, use the NWISWeb Mapper interface and zoom into your area of interest. Use the tools on the left side of screen to show active and discontinued sites.

Alternatively, use the NWISWeb Site Information page to enter the search terms of interest (for example stream name or county). These links will generate data inventories from all active and discontinued monitoring sites.

Return to FAQ List.

Q: How can I retrieve data for discontinued streamgages?

A: All approved streamflow data from active or discontinued stations is available for public access from NWISWeb. From this general link to Surface Water Data, click on Daily Data and search for your streamgages of interest.

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Q: What time increments of streamflow data are available (for example: annual, monthly, daily, or hourly) and where can I get them?

A: Streamflow data are available for different increments of time: annual, daily, hourly, and more recently 15-minute (or more frequently). Daily mean discharge is currently the officially approved and permanently archived product by the USGS. Hourly and 15-minute data are recorded at the time-increment of the field recorder and are also referred to as instantaneous or continuous data. Instantaneous data are kept publicly on-line for up to approximately a year.

All increments are available from the streamgage's NWISWeb page under the drop-down list of "Available data for this site," by selecting the preferred output format (graph, table, or tab-separated for spreadsheets) and time period. For:

Other data may be available upon request from the USGS, although these may be subject to data retrieval charges.

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Q: How can I retrieve data for many sites at once?

A: On NWISWeb, under the "Choose Site Selection Criteria" of most menus, select "Multiple Site Numbers" and on the next page enter the station identifier numbers desired. For example, see this NWISWeb page.

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Q: Why can't I retrieve rain gage or precipitation data that is more than 120-days old?

A: In order to assure precipitation is recorded as accurately as possible, the USGS recently added additional quality-assurance procedures to rain gage calibration and maintenance. In an effort to minimize streamgaging costs for the partners that help fund operations, precipitation data that was not directly funded has been removed. For additional information please contact the USGS in Minnesota at the "Webmaster" link at the bottom of this web page.

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Q: Why can't I retrieve long-term stage or gage height for streamflow streamgages?

A: There are two general reasons why long-term stage data are not available for most streamgages. First, most streamgages were installed for the purpose of determining discharge. Consequently, other than the peak stage recorded during floods (annual peak flow data, FAQ D-13 ), stage data only served as a variable required to determine discharge (FAQs C-1 and C-2 ). Second, the product or published values of discharge historically have been at intervals of daily, even though the stage data required to compute this daily discharge generally was collected more frequently, at discrete time-steps of every hour or 15-minutes (or more). These discrete values are referred to as instantaneous data. Computing discharge on a daily time step requires less processing and quality assurance of the instantaneous stage data, so erroneous data may exist in the instantaneous stage data. Finally, storage of instantaneous stage data would require a large amount of computer storage and processing time.

As computer storage and processing methods improve, the USGS is making more of it's instantaneous data available. The Instantaneous Data Archive was established to make available as much historical instantaneous data from USGS streamgages as possible. Although the site currently serves only instantaneous discharge (streamflow) data, other time-series parameters may be added in the future.

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Q: What is the difference between provisional and approved data?

A: Many USGS data retrievals indicate data are provisional. Recent or provisional data are subject to revision until they have been thoroughly reviewed and received final approval. Provisional data include real-time data relayed by satellite or other telemetry that are automatically screened from displaying improbable values until they can be verified. Provisional data may be inaccurate due to instrument malfunctions or physical changes at the measurement site. Subsequent review of data based on field inspections and measurements may result in significant revisions. Data users are cautioned to consider carefully the provisional nature of the information before using it for decisions that concern personal or public safety or the conduct of business that involves substantial monetary or operational consequences.

Approved data have been further processed and reviewed by hydrographers who apply additional corrections as needed based on field inspections and discharge measurements that were not yet available at the time of initial data posting; hydrologic comparisons, and other quality-assurance procedures.

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Q: How do I assess the accuracy of provisional streamgage data?

A: One simple way to assess the accuracy of provisional discharge data shown on a streamgage web page is to compare the continuous line-trace of the discharge hydrograph with the red asterisk that represents a recent direct measurement of the streamflow by USGS field staff. For example, see this link for Minnehaha Creek in Minneapolis on April 29, 2014.

As these discharge measurements are made over the range of flows that occur, they are used to adjust the stage-discharge relation so that streamflow in the data base are accurate. When the red asterisk representing the measured discharge is not within a few percent of the blue-line of the discharge hydrograph, expect a correction to be made soon. Note that a measurement made at high flow will not give an indication of the accuracy at medium or low flow and vice versa.

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Q: How do I import data into a spreadsheet?

A: From any retrieval you are making from NWISWeb, under "Output Options", select "Tab-separated" and submit. At the next screen, if the data appear to be what you wanted, select all (control-A for Windows or Command-A for OSX), open the spreadsheet and then select paste-special>text.

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Q: What are some data retrieving tips and shortcuts?

A: Preview the data before you download it. If you're retrieving data for loading into a spreadsheet, as in the previous example (FAQ D-11), select the option to "Display in browser" instead of "Save to a file." That way you can preview the data before downloading it. If it's not quite what you wanted, select your browser's back button, change your retrieval parameters, and display in browser again until you get what you want. Then follow the instructions in FAQ D-11 to paste the data into a spreadsheet.

Modify the URL of a retrieval you already made to retrieve same for other sites.
Most URL's for USGS data retrievals have the same format, so you can easily modify an existing retrieval or web page address to retrieve the same for other sites.

For example, change the USGS site number in the following address to quickly get retrievals of all water-quality data collected at the
Mississippi River at Brooklyn Park (site number 05288500): http://nwis.waterdata.usgs.gov/mn/nwis/qwdata/?site_no=05288500&rdb_inventory_output=file&format=rdb
and Minnesota River near Jordan (site number 05330000): http://nwis.waterdata.usgs.gov/mn/nwis/qwdata/?site_no=05330000&rdb_inventory_output=file&format=rdb

Similar edits can be done to modify retrieval dates, parameter codes, etc. For more information and more powerful retrievals, see the USGS Water Services web page.

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Q: Where can I retrieve streamflow statistics of daily-discharge, peak-flow, high-flow, floods, droughts, or low-flow?

A: USGS publishes many types of streamflow statistics. Some are user-generated, on-demand, and provisional (subject to revision). User-generated statistics are useful for comparing current streamflow conditions to historical, such as during floods or droughts, and include:

Statistics of daily mean streamflow, including the mean, maximum, minimum, and percentiles, are available from a streamgage web page on NWISWeb. On the tab at the site's NWISWeb page, under the banner "Available data for this site,"
select "Time-series: Daily statistics" from the drop-down list. At the next page, select the parameter(s) of interest, and in the Output Format pick the table statistics desired.

Monthly and annual statistics can be retrieved similarly to the previous paragraph under the banner "Available data for this site," by selecting select "Time-series: Monthly statistics" or select "Time-series: Annual statistics" from the drop-down list.

Annual peak-flow data for each streamgage is available from the streamgage's web page on NWISWeb. On the tab at the site's NWISWeb page, under the banner "Available data for this site," select "Surface-water: Peak streamflow" from the drop-down list. At the next page a graph of the data is presented, which can be changed to different "Output Formats".

Peak-flow frequency analyses of annual peak flow data described in the previous paragraph are analyzed about every 10 years in cooperation with the Minnesota Department of Transportation, whose engineers and those of other agencies use the data for culvert and bridge design (see FAQ A-1) . The latest report and analyses for 409 streamgage records in Minnesota are available in USGS SIR 2013-5110. Data are also available from an interactive web-page map.

Low-flow frequency data and analyses are available for Minnesota from the most recent low-flow report (USGS OFR 2007-1033) and interactive web-page map, in cooperation with the Minnesota Pollution Control Agency, who use the data for waste-load allocation and permitting during low-flow conditions.

Graphical and tabular summaries of streamflow conditions in context to the period of record from active streamgages are available from USGS Waterwatch. Click on the topic of interest on the left side of the web page, and try the links using your favorite streamgages site number (for example enter "05288500" to see data for the Mississippi River at Brooklyn Park). Suggested links include:

  • Duration Hydrograph which present summaries of current or past streamflow in context of the normal, low, and high streamflow for the time of year.
    Under Site Number, enter the USGS site number of interest. duration hydrograph builder. Under Site Number, enter the USGS site number of interest, for example enter 05288500, to see how recent flows at the Mississippi River at Brooklyn Park compare to historical flows for this time of year.
  • Flood Table Builder
  • which can be used to build an interactive table of streamgages presently or recently in flood, along with flood stages from the National Weather Service, days above flood stage, date of peak, rank of peak, comparison to peak of record. Select a Geographic Area, Begin and End Dates, and press Go.
  • Many other value-added summaries of streamgage data are available on the menu in the left side.
For more information contact USGS at the link at the bottom of the page.

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Q: Where can I find additional, value-added, information and analyses from this streamgage?

A: Streamgage data frequently are used for many other purposes, studies, publications, and value-added applications. The USGS in Minnesota posts links to many of the value-added products near the top of each streamgage web page in NWISWeb; click on the "twisties" to expand the list and view them.

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Q: Where can I find information on streamflow trends?

A: The USGS periodically assesses trends in streamflow for state, regional, or National needs. Below are a few examples. More USGS assessments can be found at the USGS Publications Warehouse by entering the search terms "streamflow trend"

Other reserachers also use USGS streamflow data for their assessments, as Novotny and Steffan (2006) did in this article in the Journal of Hydrology, "Stream flow in Minnesota: Indicator of climate change"

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Q: Why is discharge sometimes not displayed on a streamgage web page?

A. Most streamgages need a good relation between the stage of the stream and the discharge over the range of flows that occur to determine discharge (FAQ C-2). Two factors that invalidate that stage-discharge relation are (1) variable backwater and (2) major changes to the river channel such as caused by floods or human activities.

Variable backwater has many causes, including but not limited to ice formation, beaver dams, debris building up or vegetation growing in the stream, regulation from downstream dams, and proximity to the confluence of downstream rivers and water bodies. Backwater results in a higher stage (gage height) value than would exist during ice-free periods under the same discharge conditions. Therefore, although the values for stage remain accurate, they can no longer be used to accurately compute discharge in real time from the channel's stage-discharge rating. When field staff determine a gage is in backwater, discharges on USGS streamgage pages are turned off and the code describing the cause of the condition is substituted. For instance the code "Ice" replaces discharge data. Discharge is not displayed again until staff visit the gage and make a direct discharge measurement that confirms the stage-discharge relation. During backwater conditions, the amount of backwater varies substantially with time, temperature, and other factors. Consequently, more complex procedures involving meteorologic and hydrologic data from other streamgages in the area are required to estimate discharge at the affected streamgages. These complex procedures must be applied during review of the data by USGS personnel and, unfortunately, cannot yet be applied in real time.

Major changes to a stream channel near a streamgage can occur from natural causes like major floods that scour or fill the channel, or from human activities like bridge construction or other channel modifications. Similar to the response for backwater, these changes can also cause USGS to remove discharge from the NWISWeb streamgage page when field measurements of direct discharge indicate the existing stage-discharge relation is not correct. In this case though, the cause is that the stage-discharge relation has changed and needs to be re-defined. Consequently, discharge cannot be computed or displayed until more discharge measurements are made throughout the range of stage that occurs, in order to develop a new and accurate stage-discharge relation. The code "Rating" will be shown instead of the discharge on the streamgage web page to indicate a new rating is under development.

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Q. What is the speed or velocity of the stream I'm interested in?

A: Mean stream velocities should only be used as an approximate indication of the speed of flow through the channel and only for the location where USGS measurements are made because of limitations explained in the following paragraphs. Mean stream velocity is affected by the stage, discharge rate, and the channel slope and cross-section. Many factors affect these three variables, such as fill and scour of the channel, debris or ice accumulation, flow through vegetation, and location in the stream reach.

A few streamgages in Minnesota have instrumentation that directly measures the velocity in a part of the stream at the streamgage. This velocity is used by the USGS as an index-velocity for various purposes. The following link provides current streamgages with real-time index-velocity on NWISWeb.

For streamgages without velocity sensors, the mean stream velocity can be determined as a by-product of each discharge measurement that USGS staff make; about 6 to 12 times per year at each streamgage as described in FAQ C-2 . The mean stream velocity (in feet per second) is the total stream discharge measured (in cubic feet per second) divided by the stream cross-section (in square feet).

You can retrieve discharges measurements made at similar streamflow rates to those measured, which include the mean velocities of those measurements. From these data you can get a rough estimate of the velocity of the flow at our streamgage, which may or may not be similar to the velocities in other parts of the river. And you can retrieve tables of the mean and median daily streamflow for the periods of record of the streamgage, as well as the peak daily-mean discharge of record and lowest daily-mean discharge of record. These values give an indication of the range of flow to be expected at the sites.

Time-of-travel studies use more rigorous methods to estimate river speeds (or more accurately the average veolcity of particles in the stream) for longer river reaches. In Minnesota the most recent time-of-travel study conducted is described in USGS SIR 2004-5192 , which provides estimated travel times for seven streams tributary to the Mississippi River from St. Cloud to Minneapolis, for three flow conditions; low, median, and high. .

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(E) Floods, droughts, and current streamflow conditions in context

Q: We've had several "100-year floods" the past few years. How can that be?

A: From USGS GIP-106: the "100-year flood" is an estimate of the long-term average recurrence interval, which does not mean that we really have 100 years between each flood of greater or equal magnitude. Floods happen irregularly.

Consider the following: if we had 1,000 years of streamflow data, we would expect to see about 10 floods of equal or greater magnitude than the "100-year flood." These floods would not occur at 100-year intervals. In one part of the 1,000-year record it could be 15 or fewer years between "100-year floods," whereas in other parts, it could be 150 or more years between "100-year floods.


The USGS prefers not to use the term "recurrence interval" because it's often misconstrued ("we had a flood with a recurrence interval of 100-years last year, how can we have another one this year?"). "Annual Exceedance Probability" is a more accurate term. It is the reciprocal of the recurrence interval. So a "100-year event" has a 1-in-100 (or 1/100; or 1 percent) chance of occurring any given year; hence, a 1-percent Annual Exceedance Probability.

More information is available at the link to this USGS poster.

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Q: Where can I view previous USGS publications about floods in Minnesota?

A: The USGS publishes summaries of major floods when resources allow, often with support from other Federal and State agencies. Recent reports that document floods recorded at USGS streamgages in Minnesota include:

Other flood reports are available from the USGS Flood Information Page

Q: Where can I retrieve streamflow statistics for peak-flow, high-flow, and floods?

A: See FAQ D-13

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Q: How do I convert the stage or gage-height to an elevation above mean sea level?

A: See FAQ C-5

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Q: How does the stage or gage-height relate to the depth of a stream?

A: See FAQ C-7

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Q: How can I receive alerts from a streamgage automatically? What is WaterAlert?

A: See FAQ G-3

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Q: Where can I find information on current or past drought conditions?

A: Current drought conditions are available from the U.S. Drought Monitor Website.

Streamflow and groundwater levels can also be indicators of drought conditions. For streamflow the USGS WaterWatch web page features several tools to assess drought and low-flow conditions (roll over "Drought" on the left side of page). Many of these applications feature the ability to view historical streamflow data for previous drought conditions. For instance, raster graphs reveal periods in the record of a streamgage that were very dry or very wet. Enter the 8-digit site number of the streamgage of interest.

Shallow groundwater can also be an indicator of drought conditions or increased water use. Two sources of USGS groundwater data in Minnesota include Goundwater Watch and NWISWeb Current Conditions for Minnesota Groundwater

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Q: Where can I retrieve streamflow statistics for low-flow conditions and droughts?

A: See FAQ D-13

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Q: Is this stream unusually low or high for this time of year?

A: Provisional statistics that are useful for comparing current streamflow conditions, such as during floods or droughts, are available at the USGS WaterWatch page. Click on the topic of interest and try the links. One suggested link is the duration hydrograph builder. Under Site Number, enter the USGS site number of interest, for example enter 05288500, to see how current flows at the Mississippi River at Brooklyn Park compare to historical flows for this time of year.

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(F) Information about specific rivers, streams, and gages

Q: How did the Red River of the North get it's name?

A: That seems to be the subject of some debate, and a multi-part answer.

Encyclopaedia Britannica notes:
"After it was explored in 1732-33 by the French voyageur Pierre Gaultier de Varennes et de La Vérendrye, the river, [was] called Red because of the reddish brown silt it carries..."

In "The Streams of Minnesota" (Waters, Thomas F., 1977, University of Minnesota Press, Minneapolis, Minnesota, 371 pages), Waters writes:
"Why it should be called the "Red" is virtually lost to history. If the color had any significance, it surely must have been tangential to the nature of the water in the river. The Chippewa had a name for it in the lower reaches that suggested a flaming sunset on its flat waters, the French then called it Riviere Rouge, and the English and Americans translated that name. "But the river's waters are anything but red. Almost any other stream in Minnesota is redder than the Red River of the North. Turbid with the gray clay that settled to the bottom of glacial Lake Agassiz, the river reflects the same gray-grayish brown in low water when the clay is mixed with the brown of humic soils."

And finally, from a USGS publication (Red River of the North Basin, Minnesota, North Dakota, and South Dakota, 1993, Water Resources Bulletin of the American Water Resources Association, vol. 29, no. 4, pages 575-615).
"The name Red River was first applied by the Ojibwa Indians to the outlet of the stream of Lower Red Lake, Minnesota, which flows westward toward Grand Forks, North Dakota, and then northward to Lake Winnipeg (*Upham, 1895, p. 52, with reference to the 1852 report of the Geological Survey of Wisconsin, Iowa, and Minnesota). The idea that the Red Lake River formed the headwaters of the Red River probably relates to the fact that it is a major source of flow to the Red River of the North. The Red River of the North begins at the confluence of the Bois de Sioux and the Otter Tail Rivers. The name Red River of the North was given to the river to distinguish it from the Red River in Texas and Louisiana." *Upham, Warren, 1895, The Glacial Lake Agassiz. U.S. Geological Survey Monograph, Vol. XXV, 658 p.

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Q: Why does the Red River of the North flow north?

A: From a USGS North Dakota Water Science Center (NDWSC) web page:

"Lake Agassiz, a lake formed by melting glaciers, covered much of what is today western Minnesota, eastern North Dakota, southern Manitoba, and southwestern Ontario from about 12,500 years ago to about 7,500 years ago. Lake Agassiz virtually disappeared, leaving a few remnants like Minnesota's Upper and Lower Red Lakes and Lake of the Woods and Canada's Lake Winnipeg, Lake Manitoba, and Lake Winnipegosis. Lake Agassiz also left a fertile, flat plain that drains to the north, ultimately to Hudson Bay. The Red River flows north through this plain to Lake Winnipeg because, despite the plain being very flat, a difference in elevation exists along the route of the river, making the line between southeastern North Dakota and Lake Winnipeg slightly downhill. At the confluence of the Bois de Sioux and Otter Tail Rivers near Wahpeton, North Dakota, where the Red River begins, the elevation is 943 feet above mean sea level. The elevation of Lake Winnipeg is 714 feet above mean sea level." Click here to see a map of the elevation change.

This USGS NDWSC website has links to USGS information about the Red River and its basin, including publications, hydrographs, and pictures.

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Q: Why does the Red River of the North seem to flood so easily?

A: From USGS SIR 2005-5194:
"The Red River of the North is one of the few rivers in the United States to flow directly north into Canada. The basin flood plain lies in a glacial lakebed and is relatively flat (less than 0.5-foot drop in elevation per mile in the reach downstream from Grand Forks, North Dakota). Because of the flat basin, the shallow river channel, and the northerly flow, the timing of spring thaw and snowmelt can greatly aggravate flooding in the basin. Snow and ice in the headwaters of the Red River of the North begin to melt first, when areas downstream remain largely frozen. The melt pattern can cause ice jams to form, and substantial backwater can occur as flow moves northward toward a still-frozen river channel."

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Q: What is the history of the streamgage at the Mississippi River at St Paul?

A: The history of the streamgage at the Mississippi River at St Paul--the oldest continuous record streamgage in Minnesota with reliable records beginning in March 1892--is summarized in USGS SIR 2014-5079. This report also documents methods used to construct flood-inundation maps for the Mississippi River at St Paul.

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(G) Miscellaneous questions

Q: What is the USGS National Streamflow Information Program?

A: The USGS National Streamflow Information Program (NSIP, also known as the Groundwater and Streamflow Information Program as of 2016) is designed to provided a unified network of streamgages and streamflow information assessment to meet National, regional, and local needs. The program has five goals:

NSIP-designated streamgages must meet at least one of five Federal needs identified by NSIP:
  • Interstate and International Waters
  • - Interstate compacts, court decrees, and international treaties mandate long-term, accurate, and unbiased streamgaging by the USGS at State-line crossings, compact points, and international boundaries.
  • Streamflow Forecasts
  • - Real-time stage and discharge data are required to support flood and other streamflow forecasting by the National Weather Service and other Federal agencies across the country.
  • River Basin Outflows
  • - Resource managers need to account for the contribution of water from each of the Nation's 350 major river basins to the next downstream basin, estuary, ocean or the Great Lakes.
  • Sentinel Watersheds
  • - A network of streamgages is needed to describe the ever-changing status as it varies in response to changes in climate, land use, and water use in 800 watersheds across the country that are relatively unaffected by flow regulation or diversion and typify major ecoregions and river basins.
  • Water Quality
  • - Streamgaging stations are needed to provide the streamflow information in support of the three national USGS water-quality networks: one that covers the Nation's largest rivers; the second for intermediate-sized rivers; and the third for small, pristine watersheds.
NSIP-designated streamgages for Minnesota are shown on this NSIP web page. Streamgages are categorized on the map as active, inactive, and proposed; and which of the NSIP criteria the gages meet. In 2015, the NSIP program funded about 25 percent of streamgage operation and maintenance in Minnesota.

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Q: What are USGS Cooperative Matching Funds?

A: The USGS Cooperative Water Program is "bottom-up, on-the-ground" program designed to bring local, State, and Tribal water science needs and decision-making together with USGS national capabilities related to USGS nationally consistent methods and quality assurance; innovative monitoring technology, models, and analysis tools; and robust data management and delivery systems.

The Program provides the foundation for strong and robust water monitoring networks (quantity and quality) and supports interpretative studies - about 700 annually - that cover a wide range of issues important to the USGS water mission and that inform local, State, and Tribal water decisions.

Jointly funded programs (programs that may be matched by up to 50 percent federal dollars) are considered when the study is mutually advantageous to the USGS and localities, States and Tribes. These cooperative, jointly funded programs are reviewed and renegotiated annually to ensure that they are responsive to the needs of localities, States, and Tribes and to the national priorities of the USGS.

In 2015, the CWP program funded about 20 percent of the streamgage operation and maintenance in Minnesota.

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Q: How can I receive alerts from a streamgage automatically? What is WaterAlert?

A: The USGS WaterAlert application sends e-mail or text (SMS) messages when certain parameters, as measured by a USGS streamgages and other real-time data-collection stations, exceed user-definable thresholds.

Real-time data from USGS gages are transmitted via satellite or other telemetry to USGS offices at various intervals; in most cases, generally once per hour in Minnesota. Emergency transmissions, such as during floods, may be more frequent. Notifications will be based on the data received at these site-dependent intervals.

Instructions are available on the WaterALert website at the link above.

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Q: How can I quickly find out the stage and discharge of a river I am on? What is WaterNow?

A: The USGS WaterNow system provides a method for you to query a USGS streamgage for gage height and streamflow via email or text messaging from a cell phone. Send your request and in a few minutes you should get a reply showing the latest stage and streamflow.

To use WaterNow by email, simply send an email to "waternow@usgs.gov". In the "Subject" line put a USGS station number, such as "05288900" (for Minnehaha Creek at Hiawatha Ave in Minneapolis, MN).

For text messaging you still send your message to "waternow@usgs.gov". It's non-standard to send a text message to an email address (by default, text messages go to a phone number), so when you first start your text message, change the input method from numbers to letters or words. A "hot" key on your phone will let you change the input format. After you put in the email address hit "OK" and you will be asked to enter your actual text message. Here, you have to change your input method to numbers (the default will be letters). Just enter the USGS station number and hit "Send." (the default will be letters). Just enter the USGS station number and hit "Send." More information is available at the USGS WaterNow web page.

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Q: What is the difference between streamflow and discharge?

A: The terms are often used interchangeably. The USGS in WSP-1541-A says this about the the terms:

Streamflow - The discharge that occurs in a natural channel. Although the term discharge can be applied to the flow of a canal, the word "streamflow" uniquely describes the discharge of a surface stream course. The term "streamflow" is more general than "runoff", and streamflow may be applied to discharge whether or not it is affected by diversion or regulation.

Discharge - In its simplest concept discharge means outflow; therefore, the use of the term is not restricted to course or location, and it can be applied to describe the flow of water from a pipe or from a drainage basin. If the discharge occurs in some course or channel, it is correct to speak of the discharge of a canal or of a river. It is also correct to speak of the discharge of a canal or stream into a lake, a stream, or an ocean.

The data in the reports of the Geological Survey on surface water represent the total fluids measured. Thus, the terms discharge, streamflow, and runoff represent water with the solids dissolved in it and the sediment mixed with it. Of these terms, discharge is the most comprehensive. The discharge of drainage basins is distinguished as follows:

    Yield - Total water runout or crop; includes runoff plus underflow
    Runoff: That part of water yields that appears in streams
    Streamflow: The actual flow in streams whether or not subject to regulation, or underflow.
Each of these terms can be reported in total volumes (such as acre-feet), or time rates (such as cubic feet per second or acre-feet per year). The differentiation between runoff as a volume and streamflow as a rate is not accepted.

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Q: What is the difference between a lake and a reservoir?

A: The American Society of Civil Engineers (Task Force on Nomenclature for Hydraulic of the Committee on Hydraulic Structures of the Hydraulics Div., ASCE New York, NY: American Society of Civil Engineers, 1962, 501 pp.) uses the following definintions:

    Lakes - inland bodies of water, fresh or salt, of considerable size, occupying a basin or hollow in the earth's surface, and which may or may not have a current or single direction of flow.

    Reservoir - a pond, lake, tank, basin, or other storage space, either natural in its origin, or created in whole or in part by the building of engineering structures, which is used for storage, regulation, and control of water.
Additional information about lakes and reservoirs in general is available from the USGS Water Science School , and for lake information in Minnesota from the Minnesota Department of Natural Resources Lakes web page.

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Q: Where can I find education material about water or a glossary of water terms?

A:The USGS maintains education materials for water science here. On this page you'll also find a glossary of hydrology-related terms. The U.S. Environmental Protection Agency also has a Drinking Water Glossary.

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