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Show Wyoming and Montana and other upstream States, has created the rich bottom lands of the Missouri and the Mississippi, and built up the Mississippi Delta. Two problems must be faced as parts of the control of sediment. The first is the holding of the soil at its point of origin (discussed under water- shed management). The other is to make best use of the sediment after it has left its point of origin. Satisfactory methods of use have not been devel- oped, so measures must be taken now to eliminate the hazards they create, bearing in mind always that the sediment is of value. There are two as- pects of interest: temporary elimination of dam- age from sediment and possible methods of later beneficial use of sediments. Nature of sediment movement in the basin.- The large sediment load of the Missouri River and many of its tributaries, extensive sand movements in streams, general alluvial channel and valley characteristics, and erosion of land, have affected many of th.e projects and proposals in the basin plan. Sediments are the products of erosion carried in the streams and deposited in quiet waters. Heavy materials like sands are sorted out earlier than are the clays, some of which remain in suspension from the time they are removed from their point of origin until they are precipitated by salts in the Gulf of Mexico. Many estimates have been made as to the rate of sedimentation to be found in the Missouri River and its tributaries. The amounts vary markedly, depending -upon the source and character of the sediments, the soils and the geology of the basin, character of: the runoff, and use of the land. In the upper reaches, the coarse, heavy material carried in floodwatcrs is deposited in the flood plains and channels of the tributaries themselves as the slope of river decreases. Because of changes in gradient, some of this material may remain in the channels many years. Thus the gravel and sand of the decomposed granite of Colorado, eroded from abused mountain lands, is to "be found in great bars in the mountain reaches of tlie South Platte. Some of this material, en route foa* decades, is now entering the Denver municipal water supply reservoirs. In Montana, erosion of glacial deposits and sandstone areas is also contributing large amounts of sediment to the upper river system. This material is also to be found in the channels and flood plains of head- water rivers. Most of the load of fines in these upper streams is of small concern. After leaving the mountains, streams which trav- erse the plains pick up loads of sediments. These come from eroding cultivated and grazing lands, stream bank erosion, valley trenching, and geologic weathering. Stream bank erosion occurs generally throughout the basin. It is extensive in the Central Prairies and Great Plains. The more intensive use of the bottoms and the prevailing tendency to remove protective vegetation have resulted in severe bank erosion. It is especially serious where valley trenching is prevalent as channels meander in reaches where trenching has progressed to a stable grade. Gully erosion and valley trenching account for the greatest share of sediments. Gully erosion is accelerated by the removal of protective cover and is most severe in cultivated or overgrazed areas on rolling or rough topography. Valley trenching is the erosive process in which deepening and widen- ing of channels progresses headward in established stream systems. Much of this can be traced to land misuse. Gully erosion and valley trenching are most serious in the Central Prairies and in the eastern portions of the Great Plains. The largest and most destructive gullies are found in the loess soils of the Central Prairies and the Great Plains, where they occasionally have depths of 80 feet and widths from 200 to 300 feet (western Iowa, eastern Nebraska, northwestern Missouri, and northeastern Kansas). Gullies rang- ing from 10 to 70 feet deep are common. This type of gully often has nearly vertical sides and head. Serious land loss from gullying and valley trenching on irrigated lands occurs occasionally in the mountain and foothill valleys of the headwater areas. A significant amount of land is damaged by depo- sition of infertile material. The extent of damage is dependent upon the rate of deposition, and the 216 |