SEDIMENT CONCENTRATIONS AND VELOCITIES OF THE LOWER MISSOURI RIVER AS MEASURED BY LEWIS AND CLARK WITH COMPARISON TO MODERN CONDITIONS

Bank stabilization and hydropower development have drastically changed the character of the Missouri River below the farthest downstream dam near Yankton, South Dakota. While it has been widely recognized that these changes have resulted in decreases in suspended-sediment concentrations and turbidity, the magnitude of these decreases has not been well documented because of the small number of samples collected with questionable methods that are available before major river alterations were begun around 1914. However, the handful of semiquantitative measurements of suspended-sediment concentrations reported by Meriweather Lewis, William Clark, and John J. Audubon indicate that the decreases in suspended-sediment concentrations have been largely underestimated, and that presettlement concentrations may have been one to two orders of magnitude larger than those measured today.

Many modern-day scientists and river managers assume that channelization and a small amount of river shortening have caused large increases in river velocities. However, measurements and observations of surface velocities of the “common part of the current” made by William Clark ranged from 8 to 15 feet per second below the Platte River of Nebraska and indicate Missouri River velocities were at least as large in the presettlement period as they are today. Since 1967, when major river-development projects were completed, maximum near-surface velocities at bankfull to flood stages have ranged from between 7.5 and 13 feet per second. Qualitative and anecdotal descriptions by many other early travelers confirm that the Missouri River was an abnormally swift river. Clark and other early scientist/explorers also noted that the Missouri River actually was slower upstream from the Platte River than below, unlike most large rivers that decrease in slope and velocity in the downstream direction. Indeed, the large velocities recorded by William Clark would be required to keep the large concentrations of sediment in suspension, which he also measured.