Data describing the presence, spatial distribution, and temporal variability of nutrients, dissolved oxygen, and fecal-indicator bacteria in surface water were collected from streams in the upper Illinois River Basin from 1987-90 as part of the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) program. The largest concen- trations and loads of total nitrogen and total phosphorus were observed in streams in the urban areas of the basin. Mean annual loads of total nitrogen and total phosphorus leaving the upper Illinois River Basin accounted for 30 and 4 percent, respectively, of the input of these nutrients to the basin. Upward trends in total nitrogen concen- trations from 1978-90 were observed at three surface-water sampling stations, and downward trends in total phosphorus concentrations were observed at two stations. Median dissolved oxygen concentrations ranged from 3.4 to 12.2 milligrams per liter at eight long-term monitoring stations in the basin. During low-flow conditions, dissolved oxygen concentrations at 59 percent of the sites in the agricultural Kankakee River Basin and 49 percent of the sites in the urban Des Plaines River Basin were less than the Illinois water-quality standard of 5.0 milligrams per liter. Upward trends in dissolved oxygen concentrations were indicated at the two most downstream stations in the upper Illinois River Basin. Fecal-coliform densities at the fixed stations ranged from 1 to 45,000 colonies per 100 milliliters; stream-water samples from the Des Plaines River Basin typically had densities one or two orders of magnitude larger than samples from the rest of the Upper Illinois River Basin. Between 30 and 100 percent of the samples collected at surface-water sampling stations in the Des Plaines River Basin had densities of E.Coli greater than the Federal criteria for infrequently used full-body- contact water. Significant downward trends in bacteria densities were observed at three of the surface-water-monitoring stations.
