SUMMARY 1. Spatial patterns of benthic‐invertebrate communities were examined in the 62 900 km2 South Platte River Basin in Colorado, Nebraska and Wyoming, U.S.A., to determine major environmental factors associated with invertebrate distribution. Stable substrates were sampled semiquantitalively for invertebrates from 27 July to 7 August 1992, at twenty‐one sites. Data on physical and chemical variables were collected concurrently at each site. 2. Four site groups were identified using derrended correspondence analysis (DCA), one in the mountains and three in the plains (braided channels, tributaries near the confluence with the main stem, and sites affected by effluent from wastewater‐treatment plants). DCA axis 1 separated sites into the two major ecoregions (Southern Rocky Mountains and Western High Plains), and regression of DCA axis 1 with environmental variables indicated significant relationships primarily with slope, water temperature, specific conductance, and concentrations of organic nitrogen + ammonia and total phosphorus in surface water. Regression of DCA axis 2 with environmental variables indicated significant relationships with channel width and concentrations of nitrate + nitrite in surface water. 3. Invertebrate community composition and structure varied between ecoregions with greater number of taxa and number of insect families in mountain streams than in plains streams. Within an ecoregion, land use affected the invertebrate community. 4. Factors affecting invertebrate community distribution in stream ecosystems are scale dependent.

During August-November 1992 and August 1993, bed sediment and fish liver were sampled in the South Platte River Basin and analyzed for 45 elements in bed sediment and 19 elements in fish liver. The results for aluminum, arsenic, cadmium, chromium, copper, iron, lead,manganese, selenium, silver, uranium, and zinc are presented here. All 12 trace elements were detected in bed sediment, but not all were detected in fish liver or in all species of fish. A background concentration of trace elements in bed sediment was calculated using the cumulative frequency curves of trace element concentrations at all sites. Arsenic, cadmium, copper, lead, manganese, silver, uranium, and zinc concentrations were greater than background concentrations at sites in mining areas or at sites that have natural sources of these elements. Trace element concentrations in fish liver generally did not follow the same patterns as concentrations in bed sediment, although concentrations of aluminum and cadmium were higher in fish liver collected at mountain sites that had been disturbed by mining. Concentrations of aluminum, arsenic, cadmium, chromium, copper, iron, lead, silver, and zinc increased in bed sediments in urban areas. Iron, silver, and zinc concentrations in fish liver also increased in urban areas. Concentrations of cadmium, copper, silver, and zinc in fish liver increased in the agricultural areas of the basin. Downstream changes in trace element concentrations may be the result of geological changes in addition to changes in land use along the river.

The South Platte River Basin was one of 20 study units selected in 1991 for investigation under the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) program. One of the initial tasks undertaken by the study unit team was to review the environmental setting of the basin and assemble ancillary data on natural and anthropogenic factors in the basin. The physical, chemical, and biological quality of the water in the South Platte River Basin is explicitly tied to its environmental setting. The resulting water quality is the product of the natural conditions and human factors that make up the environmental setting of the basin.This description of the environmental setting of the South Platte River Basin and its implications to the water quality will help guide the design of the South Platte NAWQA study. Natural conditions such as physiography, climate, geology, and soils affect the ambient water quality while anthropogenic factors such as water use, population, land use and water-management practices can have a pronounced effect on water quality in the basin. The relative effects of mining, urban, and agricultural land-and water-uses on water-quality constituents are not well understood. The interrelation of the surface-water and ground-water systems and the chemical and biological processes that affect the transport of constituents needs to be addressed. Interactions between biological communities and the water resources also should be considered. The NAWQA program and the South Platte River Basin study will provide information to minimize existing knowledge gaps, so that we may better understand the effect these natural conditions and human factors have on the water-quality conditions in the basin, now and in the future. (KEY TERMS: South Platte River; water quality; water use; land use; water management; ecology.)

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