Transport and sources of sediment in the Missouri River between Garrison Dam and the headwaters of Lake Oahe, North Dakota, May 1988 through April 1991

Abstract

Sediment data were collected on and along the Missouri River downstream from Garrison Dam during May 1988, May 1989, and April 1991 to characterize sediment transport in the river. Specific study objectives were to (1) identify erosional and depositional reaches during two steady-state low-flow periods and one steady-state high-flow period; (2) determine if the reaches are consistently eroding or depositing, regardless of streamflow; and (3) determine the sources of suspended sediment in the river. Erosional and depositional reaches differed between the two low-flow periods, indicating that slight changes in the channel configuration between the two periods caused changes in erosional and depositional patterns. Erosional and depositional reaches also differed between the low-flow periods and the high-flow period, indicating that channel changes and increased streamflow velocities affect erosional and depositional reaches. The significant sources of suspended sediment in the Missouri River are the riverbed and riverbanks. The riverbed contributes to the silt and sand load in the river, and the riverbanks contribute to the clay, silt, and sand load. The contribution from tributaries to the suspendedsediment load in the Missouri River usually is small. Occasionally, during low-flow periods on the Missouri River, the Knife River can contribute significantly to the suspended-sediment load in the Missouri River. INTRODUCTION The Missouri River is the largest river in North Dakota. Mean recorded streamflow in the Missouri River at Bismarck, N. Dak., is more than 80 percent of the total mean streamflow in the State. The Missouri River is regulated by dams in Montana (Fort Peck Dam), North Dakota (Garrison Dam), and South Dakota (Oahe Dam, Big Bend Dam, Fort Randall Dam, and Gavins Point Dam; fig. 1). Of the original 390 mi of river in North Dakota, only about 90 mi are not inundated by backwater from dams. These 90 mi are located between Garrison Dam and the headwaters of Lake Oahe, which is about 20 mi downstream from Bismarck. 49° »»5° CANADA SASKATCHEWAN 105103° r 1 MANITOBA 101° 99r L 49