This report gives the results of an investigation by the U.S. Geological Survey and U.S. Bureau of Reclamation of sediment accumulation in stock reservoirs in the powder River drainage basin upstream from Arvada, Wyo. The study was made to determine the net rates of erosion in the upland areas and the effects of the reservoirs on the amount of sediment transported to the parent stream.
The climate of the area ranges from cold and humid on the high mountains to warm and semiarid on the plains. The average annual precipitation ranges from less than 15 inches on the plains to more than 27 inches in the high mountains, which have a maximum altitude of 13,165 feet. The rocks in the Powder River drainage basin range in age from Precambrian to Recent.
The 25 stock reservoirs that were used in the study have drainage areas of 0.09 to 3.53 square miles, are from 3 to 51 years old, and impound water from areas that have land slopes averaging from about 3 to 41 percent. The ratio of average reservoir capacity to drainage area ranges from about 2 to nearly 200 acre-feet per square mile.
After adjustment for trap efficiency the average annual sediment yield to the 25 reservoirs ranged from 0.04 to 1.49 acre-feet per square mile and averaged 0.50 acre-foot per square mile of drainage area. The average sediment yield from 6 drainage areas mostly underlain by shale was 0.80 acre-foot per year, 2.3 times greater than yields from the areas underlain by sandstone or sandy shales.
Correlations show that the sediment yield increased approximately as the 1.5 power of the channel density, the 0.4 power oif the shape factor, the 0.7 power of the average land slope, and the -0.25 power of the age of the reservoir.
Empirical equations for sediment yield and trap efficiency for the area studied are given.
