| OCR Text |
Show type and depth of deposit. Relatively infertile material deposited in thin layers at intervals may only slightly injure the productive capacity of the original soil, as tillage tends to mix the sediments with the soil. A thick deposit of the same material can cause much damage. Thick deposits of sandy or gravelly materials that approach or exceed plow depth make land unfit for cultivation. The great- est damage by infertile overwash occurs in the east- ern portion of the Great Plains, the western portion of the Ozark Highlands, and the Central Prairies. Damage to bottom lands by swamping, or the impairment of surface or internal drainage, is caused by sediment deposition in channels and on flood plains. In channels, large quantities of sedi- ment build up stream beds, and cause the water table to rise. Sand and sandy material, deposited adjacent to the streams, create natural levees which obstruct surface drainage from the lower lying flood plain land. Clay and fine silt, deposited on coarser textured bottom-land soils, impair internal soil drainage. This is found chiefly in the Central Prairies, where flood plain lands are intensively used and the rate of sediment deposition is high. Effects of sedimentation on water planning and use.-The effects of sedimentation influence almost every phase of water resources development on the Missouri, and may go beyond to handicap any activity touched by the river or its water. There are 209 municipalities in the watershed which treat water diverted from streams, serving approximately 2.3 million people. For them the cost of removing sediment from raw water in the basin is over a million dollars a year.17 St. Louis, although not in the basin, also takes its water sup- ply from the Missouri. Sediment deposits in road and railroad ditches, culverts, and stream channels must be removed to maintain adequate capacity for road and railroad protection. Sediment deposited on roads and rail- roads may also be a safety hazard and traffic ob- struction. Sedimentation is a cause of major damage to irrigation developments. Water diverted from clear mountain streams and from large reservoirs is reasonably free from sediment at most times of the irH. Doc. 373, 81st Gong., 1st sess. year unless carried into the system from the adjoin- ing uplands. The majority of the supply systems, however, carry heavy sediment loads. Most irri- gation developments served by the principal tribu- taries of the upper Missouri River are subject to sedimentation damage from waterways draining the slopes and uplands above them. Sediment de- posits in canals and ditches encourage the growth of weeds and aquatic plants, further reducing the capacity of canals and ditches. A principal part of the annual maintenance charges on most supply systems is for cleaning canals and ditches. Practically all of the sediment damage to drain- age projects is caused to ditches. In general, sheet erosion is the main source of these sediments. How- ever, valley trenching, deep gullies, and stream bank erosion contribute large amounts in the Mis- souri River bluff part of the Central Prairies. Dam- age is greatest along the Missouri River flood plain. As the result of a relatively abrupt change in stream gradient, large volumes of sediment are deposited where the small tributaries cross the flood plain of the Missouri River. In addition, many ditches are damaged by wind-transported materials. One of the more obvious serious sediment prob- lems is, of course, the depletion of storage capacity of a reservoir through sediment deposition. This is true of any reservoir constructed on a sediment- carrying stream. Approximately 1,140 reservoirs in the water- shed, now valued at approximately 842.5 million dollars, store water for single or multiple purposes, which may include power, irrigation, flood control, recreation, domestic water supply, and other uses. Their aggregate capacity is approximately 9.5 mil- lion acre-feet, exclusive of 13.5 million acre-feet in Fort Peck Reservoir. The annual damage to these existing reservoirs from sedimentation is esti- mated to be 3 million dollars. Some sedimentation rates as developed from reservoir surveys in the basin are shown in table 13. Over 150 reservoirs have been authorized for con- struction in the Missouri River watershed by the Federal Government. Estimated storage capacities in these reservoirs total about 122 million acre-feet. The estimated cost of these reservoirs will probably be about 2 billion dollars. 217 |
