The major sources of water in the Big Sioux River valley between Sioux Falls and Dell Rapids, S.Dak., are the Big Sioux River and the glacial outwash aquifer beneath the flood plain. The river and the aquifer are hydraulically connected.

The Big Sioux River has an average annual discharge of 246 cubic feet per second, on the average exceeds bankfull stage every 2.3 years, and has moderate basin storage.. Periods of low flow are common in the late summer, fall, and winter. A low flow of 0.1 cubic foot per second has been recorded.

The outwash aquifer is composed of permeable sand, sand and gravel, and some boulders. The aquifer underlies an area of 33 square miles and commonly has a saturated thickness of less than 30 feet.

Aquifer tests and measurements of streamflow indicate a rate of streambed infiltration of about 6 gallons per day per square foot. Analyses of streambed samples and interpretations of lithology from an electric log indicate that the streambed sediments in the diversion channel of the Big Sioux River consist largely of relatively impermeable silts and clays.

Under normal climatic conditions, most of the water pumped by the city of Sioux Falls is water that entered the aquifer by streambed infiltration.

To evaluate the importance of the aquifer in the hydrologic system, both an electrical analog model and a digital model were programmed for storage depletion alone. These models were used to determine the water-level declines in the aquifer that would result from 1 year of withdrawal at a rate of 9-10 million gallons per day. Both models indicated that the existing well field can yield water at that rate for more than 1 year.

The analog model was also used to determine the maximum average daily yield of the aquifer-river system for a 2-year period modeled for the minimum recorded stream discharge and for streambed infiltration as the only recharge. The maximum average yield under these conditions would be less than 24 million gallons per day. Most of the water pumped during this period would be from aquifer storage.

Water from the river is generally less mineralized, softer, and easier to treat than ground water. Water pumped from wells near the river is similar in quality to the river water, but does not have the objectionable odors or tastes often present in water from the river.