A water-quality, data-collection network on a 26.2-mile reach of Cedar Creek in west-central Illinois was operated from May through December 1985 and from March through October 1986. The study reach drains a 60.8-square-mile area of predominantly agricultural land. However, the city of Galesburg contributes combined- and storm-sewer discharge to the creek that can affect the water quality. This report presents the streamflow and water-quality data collected and describes the methods and instrumentation used. Continuous discharge data were determined at eight gaging stations--five on the main stem and three on tributaries of Cedar Creek. Flows ranged from 0 to 1,200 cubic feet per second. A total of 8 storm sewers and 51 combined sewers were monitored or inspected as part of the study. Continuous precipitation data were collected at five sites. The maximum hourly precipitation intensity during the study period was 1.97 inches per hour. The maximum total precipitation for a storm during the study period was 4.16 inches. Sediment-oxygen-demand rates were measured at 45 locations in the creek. The measured sediment-oxygen-demand rates ranged from 0.4 to 9.1 grams of oxygen per square meter per day. Ninety-seven bottom-material samples were collected and analyzed for chemical oxygen demand, percentage of volatile organic constituents, and concentrations of total organic plus ammonia nitrogen, arsenic, cadmium, chromium, copper, iron, lead, manganese, mercury, and zinc. Traveltime and reaeration rates were determined for 15 subreaches of Cedar Creek during various flow conditions. The reaeration rates ranged from 3.10 to 20.8 per day. Three intensive diel (24-hour) data collections were conducted during low-flow conditions. Data included dissolved oxygen concentration, biochemical oxygen demand, nutrient concentrations, and metal concentrations. During storm-related high-flow conditions, water-quality samples were collected at 5 main-stem sites, 3 tributary sites, the Galesburg wastewater treatment-facility outfall, and 15 sewer outfalls. These samples were analyzed for nutrients and metals, suspended solids, chemical oxygen demands, and biochemical oxygen demands. This report describes methods for measuring stage, discharge, precipitation, sediment oxygen demand, traveltime, and reaeration rate. Collection methods for bottom-material samples, diel water-quality data and samples, combined-sewer overflow frequency and duration, and storm- related sampling of combined sewers, storm sewers, and streams are described. A brief discussion of streamflow computation methods also is presented.