During May through August 1993, water-quality samples were collected twice from selected sites in Rathbun Reservoir, Iowa, Clinton and Pomona Lakes, Kansas, and Harlan County Reservoir, Nebraska. Samples were analyzed for selected physical and chemical properties, bacteria, major ions, nutrients, selected metals, total organic carbon, selected herbicides, and chlorophyll-a and -b. During May through August 1993, precipitation at all four reservoirs exceeded mean precipitation at nearby long-term precipitation gages; precipitation ranged from 7.76 inches above the long-term mean at Pomona Lake to 12.62 inches above the long-term mean at Rathbun Reservoir. Reservoir water-surface elevations exceeded flood-pool elevation by 4 feet in Clinton Lake, by 19 feet in Pomona Lake, and by 2 feet in Harlan County Reservoir in July. Thermal stratification of water occurred at one site in Rathbun Reservoir in August, at two sites in Clinton Lake in May, at one site in Pomona Lake in June, and at one site in Harlan County Reservoir in June. Total triazine herbicide concentrations in water samples from all four reservoirs ranged from 0.2 to 19 micrograms per liter and were largest water samples from Rathbun Reservoir in June. Concen- trations of atrazine exceeded the U.S. Environmental Protection Agency's Maximum Contaminant Level of 3.0 micrograms per liter for drinking water in at least one sample each from Rathbun Reservoir, Clinton Lake, and Pomona Lake. Concentrations of cyanazine exceeded the U.S. Environmental Protection Agency's Maximum Contaminant Level of 1.0 microgram per liter in water samples from Rathbun Reservoir.