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Show dollars, plus dredging and maintenance, including the cost of energy used. On the basis of the anal- ysis the desilting costs added an average of one mill per kilowatt-hour to the cost of generation. This analysis, and the Garrison estimate, indicate that reservoir storage of sediment at present is economi- cally more feasible than other desilting operations at the reservoir site. The principal known way of alleviating sediment inflow into reservoirs is to lo- cate the chief sources of the sediment and apply preventive treatment. An example of this is the study of the sediment problem at Boysen Dam on the Big Horn River. A sediment program for the Wind River Basin has been begun, involving study of land types, suscepti- bility to erosion, and sediment gaging of the main stream and principal tributaries. When it was found that one tributary produced a large portion of the sediment passing Boysen, the sampling proce- dure was extended to streams in that basin. It was determined that about 40 percent of the sedi- ment at Boysen Dam comes from an area of 347 square miles on Five Mile Creek, which is only about 5 percent of the total area of 8,000 square miles above Boysen. Further investigation revealed that only about 10 percent of the sediment in this stream came from the range and irrigated land, and about 90 percent from stream bed and banks.18 However, bank and stream bed erosion have been greatly increased because the return flow from the Riverton Irrigation Project has caused the average stream flow to increase over 30 times. (Natural flow was 3,000 acre-feet per year* pres- ent flow is 100,000 acre-feet.) Measures of control are now being studied. They include canal diver- sion, upstream storage, and use of vegetation in the stream channel. No estimate of costs is yet available, but there are indications that means may be found to achieve better control of sediment near its source. Many estimates have been made as to the rates at which sedimentation will take place in proposed reservoirs. These estimates will be revised, some probably radically, as better data on sediment loads are acquired and the trap efficiency of various res- ervoirs becomes known. The determination of the sources of sediment has not yet been under- taken, and the use of engineering devices or debris structures to reduce the loss of reservoir space from sedimentation has not been planned, even in areas where sedimentation rates are known to be exces- sive. Associated with the depletion of storage capacity in a reservoir is the sediment deposition upstream from a reservoir, which in some instances causes serious extended backwater problems.19 An ex- ample of this type of sediment problem is found at Williston, N. Dak. Local residents fear the effects of sedimentation in the Garrison Reservoir. This problem was recognized and remedial measures for the protection of Williston are a part of the project. Levees and related works would fully protect Wil- liston and adjacent irrigation areas, not only dur- ing initial conditions but also during the advanced stages of aggradation. Another problem of reservoir construction on alluvial streams is the degradation of the stream bed downstream from reservoirs. It is recognized that the construction of reservoirs on silt-carrying streams interferes with the previous natural move- ment of sediment, and will change the previously existing stream balance. Clear water discharged from the reservoirs will tend to regain its sediment load by eroding the channel below the dam. The total amount of sediment moved by the stream therefore may be increased considerably for a period of years, although it is expected that discharge of sediment at the mouth eventually will be less than it has been in years past. A further example of the sediment problem ap- pears in the possibilities for future slack-water navi- gation on the lower river. Existing studies indicate that structures necessary for slack-water navigation would under present conditions rapidly fill with sediment and be ineffective for the purpose in- tended. In part because of this it has not appeared practicable to provide for slack-water navigation in present plans. However, in the future, if the uses for water should exceed the supply, sediment con- trol may be important in avoiding conflicts among water uses, and promoting the most effective use of water resources. River-borne sediments also produce some bene- ficial results. Along the lower Missouri, bank sta- bilization works being constructed to improve the river for navigation contract the waterway to some extent. Sedimentation landward of the dikes and revetments creates additional valley lands useful for agriculture or grazing. Sediment also is useful 18 Sloan, W. G., "The Silt Problem in the Missouri River Basin," Address before Missouri Basin Inter-Agency Com- mittee, June 29, 1950, p. A-9. 19 I. e., water levels higher than those that would other- wise exist in the affected reaches. 219 |
