| OCR Text |
Show An extremely heavy sediment load is character- istic of the Missouri River and many of its tribu- taries. It contributes more sediments and has a greater average concentration of sediment than any other tributary of the Mississippi. This charac- teristic is reflected in the common name by which the Missouri is known-the Big Muddy. These sediment loads have considerable bearing upon res- ervoir construction and other works in the basin. The solid burden carried by the streams is de- rived from many sources: farm lands, especially cultivated or bare slopes; woodland and brush land on which the vegetal cover affords inadequate pro- tection; grasslands, especially the over-grazed ranges; areas of loose, unconsolidated soils such as those of the Dakota badlands; earth roads, ditches along roads, unprotected cuts and fills along high- ways and roads; and especially the erosion of stream channels and banks. The largest sediment con- tributors to the Missouri are the Yellowstone, Little Missouri, Cheyenne, Bad, White, Platte, Kansas, and Grand (Missouri). There is wide variation in the amount of sedi- ments discharged by the Missouri. Three-fourths of its annual average sediment load of 200 million tons or over is discharged during April, May, June, and July; the largest amount is discharged during June. In flood years the sediment load carried into the Mississippi is five times that carried in low- flow years. The sediment is of two kinds: the coarser ma- terial, such as sands, which are rolled along the bottom in a semifluid condition, and the fine ma- terial, mostly clay, carried in suspension. This fine material is continually being carried out of the basin, while the heavier is discharged chiefly during floods. Normally the river carries heavy sediment loads from one place to another throughout its length. The larger particles are deposited as ve- locity decreases from place to place. As the velocity increases with an increase in flow, particles are picked up again and carried on, so that at high stages vast quantities are carried, deepening the river and eroding the banks. As the flow is re- duced, deposition once more takes place, filling the river bed and depositing bars. Thus rapid changes are continually occurring in the character of the river bed. Reduction of the sediment load is a major factor in river improvement. As far as possible the soils that provide the sediment should be held in place. Some small isolated areas are thought to be the source of a major part of the sediment, even in large tributaries. Some sources have existed for years. One of the major sediment sources is the Yel- lowstone, and for that stream the largest source is the Big Horn, with the Powder a secondary source. In flood the Powder River is perhaps the most heavily loaded of all streams in the basin. On the Big Horn, a small tributary of the Wind River is the principal source of sediment. With proper man- agement this area could probably be controlled. Even with control, the Big Horn will continue to carry heavy sediment loads until the sediment de- posited along its course has been moved, and the channel stabilized for the clearer water flow. On these rivers the Yellowtail Reservoir on the Big Horn will temporarily prevent sediment from reaching the Yellowstone. The Moorhead Res- ervoir on the Powder River will do the same. At present, controlled sedimentation appears to be the only method for stabilizing the lower reaches of the Missouri. 173 |
