The 15,300-square-mile lower Kansas River Basin in Kansas and Nebraska was investigated, as one of the pilot study units of the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program, to address a variety of water-quality issues. This report describes sanitary quality of streams as defined by concentrations of dissolved oxygen (DO) and densities of a fecal-indicator bacterium, Escherichia coli (E. coli). Sixty-one surface-water sampling sites were chosen for this investigation. Synoptic surveys were conducted in July 1988, November 1988, March 1989, and May 1989 to define the concentrations and diel and seasonal variability in concentrations of DO. Synoptic surveys were conducted in July 1988 and July 1989 to define densities of E. coli. Ancillary data included measurements of specific conductance, pH, water temperature. barometric pressure, and concentrations of nutrients, total organic carbon, chlorophyll, and suspended sediment. Surveys were conducted during stable-flow, dry-weather conditions. During the July 1988 synoptic survey for DO, emphasis was placed on the measurement of DO under maximum stress (high water temperature, low streamflow, and predawn conditions). Of 31 sites sampled just before dawn, 5 had DO concentrations less than the 5.0-milligrams-perliter, l-day minimum warmwater criterion for early life stages as established by the U.S. Environmental Protection Agency (USEPA), and 4 of these 5 sites had concentrations less than the 3.0-milligrams-per-liter criterion for all other life stages. For all four synoptic surveys, a total of 392 DO determinations were made, and 9 (2.3 percent) were less than water-quality criteria. Concentrations of DO less than water-quality criteria in the study unit are localized occurrences and do not reflect regional differences in DO. The most severe DO deficiencies are the result of discharges from wastewater-treatment plants into small tributary streams with inadequate assimilation capacity. Algal respiratory demand in combination with reduced physical reaeration associated with extreme low flow probably also contributes to temporary, localized deficiencies. Densities of E. coli were determined at 57 surface-water sampling sites during the syn- optic survey in July 1988. Results indicate large regional differences in E. coli densities within the study unit. Densities orE. coli in water at 19 sites in the Big Blue River subbasin, exclusive of the Little Blue River subbasin, ranged from 120 to 260,000 col/100 mL (colonies per 100 milliliters), with a median density of 2,400 col/100 mL. Densities at the 11 sites in the Little Blue River ranged from 100 to 30,000 col/100 mL, with a median density of 940 col/100 mL. Densities at the 27 sites in the Kansas River subbasin ranged from less than 1 to 1,000 col/100 mL, with a median density of 88 col/100 mL. Densities at 84 percent of the sites in the Big Blue River subbasin exceeded the USEPA E. coli criterion of 576 col/100 mL for infrequently used full-body contact recreation, and 53 percent exceeded the 2,000 cot/I00 mL fecal coliform criterion for uses other than full-body contact established by the Kansas Department of Health and Environment. Densities at 73 percent of the sites in the Little Blue River subbasin exceeded the 576 col/100 mL E. coli criterion, and 36 percent exceeded the 2,000 col/100 mL fecal coliform criterion. Densities at one of the sites in the Kansas River subbasin exceeded the 576 col/100 mL E. coli criterion, and none exceeded the 2,000 col/100 mL fecal-coliform criterion. The largest densities of E. coli in the study unit were the result of discharges from municipal wastewater-treatment plants; however, densities in the Big Blue and Little Blue River subbasins were generally larger than those in the Kansas River subbasin. These larger densities in the Big Blue and Little Blue River subbasins may have been the result of irrigation return flow from fields where manure was used as a soil