Water-Quality and Ecological Issues

Protection of water quality in rivers and in aquifers in the study area is an important concern. The greatest effects on hydrology, water quality, and aquatic communities occur in areas where the disruption to the natural land cover is substantial.

Water quality is affected less in the forested areas of the study unit (Upper Mississippi River and St. Croix River Basins) than in other areas of the Study Unit due to minimal land-cover disturbances. Water quality in the forested areas may affected by minor applications of herbicides at small farms, tree farms, and in lakes (for weed reduction); discharges of waste-water effluent; leaks from septic systems; local stream-channel disturbances from forestry; and localized draining of wetlands. These activities result in small increases in nutrient, pesticide, suspended-sediment, and bacteria concentrations relative to natural conditions. In general, there are few physical modifications to stream channels in forested areas with the exception of some areas that are used are used for commercial forestry practices. Aquatic communities are relatively undisturbed in forested areas due to minimal contaminant sources and minimal physical alterations to surrounding land use.

In agricultural areas (most of the Minnesota River Basin and parts of the Mississippi and St. Croix River Basins),water-quality concerns include degradation from point and nonpoint sources of sediment, nutrients, and pesticides, and physical alterations of streams through artificial drainage systems, channel straightening, and removal of riparian vegetation.

Riparian zones are the interface between streams and the surrounding watershed. Wooded riparian vegetation can impede surface runoff and stabilize stream banks thereby reducing the delivery of sediment into streams. Reduced volumes of sediment delivered to streams may have beneficial effects on aquatic resources by reducing the amount of siltation on fish and invertebrate habitat areas. Reduced sediment in streams increases the visibility for animals that are site predators and increases light penetration that enhances the growth of algae. Wooded riparian vegetation absorbs and reflects solar radiation, resulting in greater thermal stability and decreased light availability for primary production. Wooded riparian vegetation increases habitat complexity by supplying woody debris, which contributes to greater variation in channel and provides habitat for invertebrates, especially in low-gradient streams.