Stream water-quality data from part of the Upper Mississippi River Basin Study Unit (Study Unit) from 1995 through 1998 was used to describe the distribution of nutrients, chlorophyll a, phytoplankton, and suspended sediment; and the influence of natural and anthropogenic factors on reported concentrations, loads, and yields. During the study period, streamflows generally were near to greater than average. Agricultural land cover, particularly on tile-drained soils, had the most substantial influence on nutrients, chlorophyll a, and suspended sediment in the Study Unit. The greatest concentrations and yields of total nitrogen, dissolved nitrite plus nitrate nitrogen, dissolved nitrite nitrogen, total organic plus ammonia nitrogen, total phosphorus, and suspended sediment were measured in a stream representing agricultural land cover on tile-drained soils. Total nitrogen yields also were about 6 times greater in a stream representing agricultural land cover on tile-drained soils than in a stream representing agricultural land cover on naturally welldrained soils.

Urban-residential land cover also influenced nutrients and suspended sediment in streams in the Study Unit. Concentrations and yields of total nitrogen, dissolved nitrite plus nitrate nitrogen, total organic plus ammonia nitrogen, total phosphorus, dissolved orthophosphate phosphorus (orthophosphate), and suspended sediment in streams representing urban-residential land cover were less than those representing agricultural land cover on tile drained soil, and greater than those on forested land cover.

Nutrients and suspended sediment in the Mississippi, Minnesota, and St. Croix Rivers generally reflect the influence of primary land cover in drainage areas of streams tributary to these rivers. In the Mississippi River, total nitrogen, dissolved nitrite plus nitrate nitrogen, dissolved ammonia nitrogen, total phosphorus, dissolved orthophosphate, and suspended-sediment concentrations and loads increased substantially downstream of the confluence with the Minnesota River at Hastings, Minnesota. Greater concentrations and loads of dissolved orthophosphate in the Mississippi River at Hastings, Minnesota probably were caused by wastewater discharges from the Twin Cities Metropolitan Area (TCMA) and contributions from the Minnesota River. The median dissolved ammonia nitrogen concentration at the Mississippi River at Hastings, Minnesota, based on data collected during this study, was about one-half of the median concentrations previously reported for 1984-93, which can be attributed to reduced ammonia concentrations in municipal wastewater discharged from the TCMA.

In the St. Croix River, total nitrogen, dissolved nitrite plus nitrate nitrogen, and total organic plus ammonia nitrogen concentrations were significantly greater at St. Croix Falls, Wisconsin compared to near Danbury, Wisconsin. All nutrient and suspended-sediment loads and yields in the St. Croix River increased in the downstream direction from near Danbury to St. Croix Falls, Wisconsin.

Most sites had pronounced seasonal variations in dissolved nitrite plus nitrate nitrogen and dissolved ammonia nitrogen concentrations. At most sites, dissolved nitrite plus nitrate nitrogen concentrations were greatest in the winter and spring and least during the summer and fall. In contrast, the greatest dissolved nitrite plus nitrate nitrogen concentrations in the Little Cobb River near Beauford, Minnesota; Minnesota River near Jordan, Minnesota; and Mississippi River at Hastings and Red Wing, Minnesota occurred during the spring and summer. These seasonal variations in dissolved nitrite plus nitrate nitrogen concentrations may be the result of nitrogen cycling in the soils, as well as crop uptake and hydrologic conditions. The greatest concentrations of dissolved ammonia nitrogen at all sites occurred in the winter and spring. The maximum contaminant level for nitrate of 10 milligrams per liter (mg/L) as nitrogen set by the U.S. Environmental Protection Agency (USEPA) for drinking water was exceeded in 20 percent of the samples analyzed from the Little Cobb River near Beauford, Minnesota, and in 11 percent of the samples analyzed from the Minnesota River near Jordan, Minnesota.

The greatest chlorophyll-a concentrations and algal abundances generally were measured in the Little Cobb River near Beauford, Minnesota; Minnesota River near Jordan, Minnesota; Mississippi River at Hastings, Minnesota; and the Mississippi River at Red Wing, Minnesota. Greater concentrations and algal abundances at these sites may have been the result of increased nitrogen and phosphorus concentrations. Total phosphorus concentrations at these sites most frequently exceeded the goal of 0.1 mg/L set by the USEPA to prevent eutrophication. Phytoplankton communities at these sites primarily were dominated by blue-green algae during the summer of 1996. In contrast, at most of the other sites, the phytoplankton community was dominated by diatoms.