Fluvial sediment of the Mississippi River at St. Louis, Missouri

An investigation of the fluvial sediment of the Mississippi River at St. Louis, Mo. , was begun in 1948. Most data have been obtained only to determine the daily suspended-sediment discharge and the particle-size distribution of suspended sediment and bed material, but a few data have been obtained to study the flow resistance, the vertical distribution of sediment and velocity, and the bed-material discharge. The flow of the Mississippi River at St. Louis is made up of the flows from the Missouri River, which had an average flow of 79,860 cubic feet per second for 1897-1958 at Hermann, Mo. , and from the upper Mississippi River, which had an average flow of 91,890 cubic feet per second for 1928-58 at Alton, 111. The Missouri River is partly controlled by reservoirs that had a total capacity of 90,300,000 acre-feet in 1956, and the upper Mississippi River is prHly controlled by lakes and reservoirs that had a total capacity of 4,890,000 acre-feet in 1956. The flows of the Missouri and upper Mississippi Rivers have not become mixed at St. Louis; so the river has a lateral gradient of suspended-sediment concentration. The concentration near the west bank has been as much as 2,400 parts per million greater than the concentration near the east bank. Suspended-sediment discharges from April 1948 to September 1958 ranged from 4,250 to 7,010,000 tons per day and averaged 496,000 tons p? r day. Mean concentrations for water years decreased steadily from 1,690 parts per million in 1949 to 403 parts per million in 1956, but they increased to 756 parts per million in 1958. Effects of new reservoirs in the Missouri River basin on the concentration have been obscured by the close relation of concentration to streamflow. Measured suspended-sediment discharge through September 19*8 averaged 47 percent clay, 38 percent silt, and 15 percent sand. Variations of particle size were due mainly to differences in the source areas of the sediment. Most of the bed material in the main flow was between 0.125 and 1.000 millimeter in diameter. The average of median diameters was related to the discharge for periods of 1 year and longer. Geometric quartile deviations of the bed material ranged from 1.1 to 2.5 and averaged 1.5. The mean elevation of the bed had a range of almost 10 feet end was related to the median diameter of bed material by the regression equation h b = 363.0 7.8 dso for which the standard error of estimate was 0.91 foot. The resistance to flow as measured by Manning's n ranged from 0.024 to 0.041 and was related to the discharge and mean velocity but not to the shear velocity. Normal dune height is 2-8 feet, and average dune length is about 2 rO feet. When the resistance to flow was low, much of the bed was fairly flat; a few dunes were 1 2 FLUVIAL SEDIMENT, MISSISSIPPI RIVER, ST. LOUIS present, but they were much longer than the average. For a giver discharge during individual rises in stage, the gage height was lower for increasing discharge than for decreasing discharge even though the bed elevation was higher. The changes in gage height were not caused by changes in energy gradient due to changing discharge, by channel storage between the gage and the measuring section, nor by return of overbank flow; but they were probably caused by a combination of changes in roughness due to changing bed configuration and of changes in turbulence constant due to changing sediment concentration. Turbulence constants (Von Karman's k) computed from velocity measurements at 5-10 points in the vertical and from routine velocity measurements at 2 points in the vertical averaged 0.35 and 0.33, respectively. The exponent zi of the vertical distribution of concentration for different size ranges varied with about the 0.77 power of the fall velocity. Except for the difference between the theoretical variation and the actual variation of z\ with changing fall velocity, the theoretical equation for the vertical dist-ibution of sediment concentration seems to apply reasonably well for the Mississippi River at St. Louis. The accuracy of methods for computing bedload discharge and bed-material discharge could not be evaluated directly but was evaluated from tie relations of computed bedload discharge or bed-material discharge to the measured part of the bed-material discharge and to mean velocity. These relations indicated that bedload discharges can be computed most accurately by using the modified Einstein, Schoklitsch, and Meyer-Peter and Muller methods as comoared with the Kalinske and Bagnold methods. The relations indicated that bed-material discharges can be computed most accurately by adding the unmeasured sediment discharge from Colby's procedure to the measured part of the bed-material discharge when data on velocity, concentration, and particle-size distribution are available, or by using Straub's equation when such data are not available. Measured suspended-sediment discharge averaged about 95 percent of the total sediment discharge, and bed-material discharge averaged about 14 percent.