Water-quality problems in the Big Sioux River have been maintained by the severe depletion of oxygen resources and by excessive ammonia concentrations downstream of Sioux Falls, SD. Low-flow conditions occur periodically in the in the Big Sioux River, thus increasing its vulnerability to the effects of carbonaceaous and nitrogenous waste loads. A major source of these wastes is the Sioux Falls wastewater treatment plant. The report includes water-quality investigations in the Big Sioux River and selected tributaries prior to and during critical conditions of low flow and ice cover.