An investigation of the chemical quality of surface waters and of the sedimentation in the Grand River drainage basin by the U.S. Geological Survey began in 1946. The chemical quality of the water was studied to obtain information on the nature and amounts of dissolved solids in the streams and on the suitability of the water for domestic, industrial, and irrigation uses. Sedimentation was studied to determine the quantity of sediment that is transported by the streams, the particle sizes of the sediment, and the probable specific weight of the sediment when deposited in a reservoir.

The basin is underlain by consolidated sedimentary rocks of Cretaceous and Tertiary age; along the Grand River and its tributaries thefe rocks are mantled by alluvium of Quaternary age. The Hell Creek and Fort Union Formations underlie about 4,700 of the 5,680 square miles of drainage area. The climate of the basin is semiarid and is characterized by tot summers and cold winters. Mean annual runoff is about 53 acre-feet per square mile of drainage area and is equal to about 7 percent of the mean annual precipitation. The highest streamflows on the Grand River and major- tributaries are caused by melting of snow in March and April. Streamflow is extremely variable from year to year.

Most of the surface waters in the basin are of the sodium sulfate or sodium bicarbonate type. High percent sodium is typical of almost all the surface water's. The streamflow-quality patterns of the Grand River and its two forks are very similar; dissolved-solids concentration, which usually c'oes not exceed 3,000 ppm, is maximum during low-flow periods.

The water in Shadehill Reservoir became stratified during the flood inflow of 1952; about 75 percent of the flood water, which was of good qutlity, passed through the reservoir. The quality of the water became almost uniform throughout the reservoir the latter part of July 1952. After the specific conductance became relatively stable in 1956, it fluctuated from abrut 1,300 to 1,600 micromhos per centimeter and was between 1,400 and 1,500 rnicromhos per centimeter most of the time.

During the representative period July 1937 to June 1950 the quantity of dissolved solids passing the station near Wakpala was estimated to have been about 140,000 tons per year. Yields computed for different parts of the basin ranged from about 22 to 32 tons per square mile.

Except for sulfate, concentrations of chemical constituents usually do not exceed the maximum concentrations recommended for domestic supplies. The rather high dissolved solids and hardness of most of the surface waters prevent the use of these waters for most industrial purposes unless the quality is improved by treatment.

Classified for irrigation use according to its specific conductance and sodiumadsorption-ratio, the water stored in Shadehill Reservoir has a high salinity hazard and a medium sodium hazard. The water can be used safely forsustained irrigation on soils of the proposed irrigation unit if adequate teaching is practiced and if gypsum or some other calcium compound is added to the water or land during the high sodium cycle.