Recharge Processes on the Trout Creek and Cottonwood Creek Alluvial Fans, North-Central Nevada

by Russ Plume (rwplume@usgs.gov)

Trout and Cottonwood Creeks are ephemeral tributaries of the Humboldt River in north-central Nevada (Figs. 1 and 2). Because they are typical of numerous streams that drain small watersheds in the Humboldt River Basin, the site of these two watersheds was chosen as one of five study sites for ground-water recharge processes on alluvial fans by the USGS Southwest Ground-Water Resources Project (http://az.water.usgs.gov.swgwrp/Pages/Overview.html). The Humboldt River Basin Assessment (http://nevada.usgs.gov/humb/) also supports the Trout-Cottonwood study. The objectives of the study, which runs from 1999 to 2003, are: (1) to quantify ground-water recharge that results from mountain block runoff along the two stream channels and from precipitation that falls directly on the alluvial fan, and (2) to quantify subsurface ground-water flow from the mountain block into adjacent alluvial-fan deposits.

Altitudes of both watersheds range from about 1,340 m (4,400 ft) along the edge of the Humboldt River flood plain to 2,500 m (8,200 ft) in the headwaters areas on Battle Mountain. From the headwaters, each stream flows northwest and north in a rugged canyon that is incised into faulted and fractured cherts, shales, siltstones, sandstones, quartzites, and subordinate limestones of Paleozoic age. At the mountain front, both streams turn to the northeast and flow about 11 km (7 mi) across a broad, gently sloping alluvial fan to the Humboldt River flood plain. During years of at least average winter precipitation on Battle Mountain, flows of Trout and Cottonwood Creeks may reach the Humboldt River in late spring to early summer. During drought years, the flows may not even reach the alluvial fan-mountain block contact.

Measurement of streamflow, soil and water temperature, soil moisture, and the chloride content of the unsaturated zone will provide four independent estimates of recharge rates on the alluvial fan. Mining companies have installed several monitoring wells in the mountain block and alluvial fan. Water levels, hydraulic properties, and chloride analyses of ground water from these wells will be used to estimate subsurface flow across the alluvial fan-mountain block contact.

In spring of 2000, the study is in the early phase of data collection. Because of funding limitations, much of the study emphasizes Trout Creek. Miscellaneous measurements of streamflow in late spring and early summer 1999, and in 2000, provided an initial indication of the potential timing and magnitude of flows in both streams, and of flow losses across the alluvial fan. Present data collection includes: (1) continued streamflow measurements at nine sites; (2) continuous measurement of streamflow at five gaging stations; (3) measurement of soil-surface and runoff temperature using Onsett temperature probes at 13 sites; (4) measurement of infiltration temperatures at depths of 10, 20, 50, 100, 150, and 200 centimeters using thermocouples at four sites; and (5) collection of meteorological data and chloride dryfall and wetfall at two weather stations. Data collection will continue for the next two runoff seasons (2001-02). The report of study findings will be completed and released by 2003.

For more information, contact Dave Prudic (775-887-7611) or Russ Plume (775-887-7612).

	Figure 2. —Study area data collection sites. 2.--