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Show In general, the high mountain area is not a major sediment-producing area. However, even in the mountainous area, cover abuse by livestock has reached such a stage that serious erosion and corresponding sediment production have occurred. Kannah Greek in the Grand Mesa National Forest is an example. This creek, the water supply source for Grand Junction, has gone through the same general history as other sections of the basin. The area started to deteriorate around 1890. Now 70 percent of the drainage area is in an active gully and sheet erosion stage. Good watershed cover has been replaced by sagebrush and weeds. Almost 92 percent of the area is now unsatisfactory from a watershed standpoint, 60 percent of the area classed as getting worse, and the remainder is merely stand- ing still.84 Deterioration has reached such a stage that reductions in range use by both livestock and big game are essential. Other areas of the western slope in Colorado show similar conditions on forest lands, but the problem is not as acute as on the open range at lower elevations. Most of the eroded material comes from the semi- arid portion of the basin. (See problem G-2.) Here is where the greatest damage has been caused. The great nomadic herds which wandered over the basin are gone. Now, although the area is still over- grazed, feAver head of livestock are able to keep the land impoverished. About 80 percent of the area (the semiarid portion) produces most of the sedi- ment and only about 20 percent of the water. It is in this area that great gullies have been cut, and surface soil washed away. It is here also that most geological erosion has occurred, although much of the geologically eroded material was deposited on fans in small drainage ways and did not reach the river. Valley erosion is now cutting into an accumula- tion of geologically eroded material, as in the Kanab Creek watershed above Elbow Dam site near Kanab, ITtah. Measurements indicate the annual sediment production is from 210 to 550 acre-feet. In this watershed all channel ways are incised and connected . Virtually all are cutting headward to- ward new sediment sources. Unless this cutting is stopped, large deposits of sediment in fans at the mouth of numerous canyons emerging from Bryce Canyon National Park will be tapped. If this oc- curs sedinxent production will greatly increase. The 11,000 acre-foot Elbow Reservoir, with an estimated " Study of Kannah Greek Watershed, 1948, U. S. Forest Service. trap efficiency55 of 92 percent, would lose 60 per- cent of its capacity in 17 years.56 Effects of sedimentation.-Loss of storage capac- ity is not the only loss suffered by reason of sedi- mentation. Shoaling of the reservoir results in in- creased evaporation, and exposure of deposited! sediments in the upper reaches permits invasion of water-loving plants whose transpiration rate ex- ceeds that of evaporation. Deposits of sediments in channels and drainageways also become oc- cupied with water-wasting plants. A deposition of 7,800 acre-feet of sediments in the Roosevelt Reservoir between 1925 and 1935 caused an increase in the surface area of about 450 acres. The evaporation from this increased area amounted to some 2,500 acre-feet a year.57 The sediment problem not only is related to the larger dams, but it also affects many small stock ponds throughout the entire basin. Some 1,500 of these have been built to provide supplemental water to livestock on both public and private lands. Many of these facilities are threatened by sediments, and their maintenance is difficult and costly.58 Sedimentation has also been serious in the Gila River. It has blocked some drainage outlets, filled canals, and so filled in the river bed as to make direct diversion at times an impossibility.59 Sedimentation promises to be a factor affecting recreation developments. In mountain areas, sedi- ments have caused swamping of some desirable sites.60 Sediments in some reservoirs will affect recreation facilities and shorten the period some sites may be used. This problem will be acute in the upper reaches of the proposed Bridge Canyon Lake.61 Sedimentation is also a secondary problem in min- ing developments, because of instability of owner- ship. During profitable periods, miners construct impounding dams to hold tailings. When the mine shuts down, the supporting dams are not main- M Trap efficiency is the capacity of a reservoir to arrest and retain sediment. M E. M. Thorp, Proceedings, Federal Inter-Agency Sedi- mentation Conference, Salt Lake City, 1950. w T. Maddock, Bureau of Reclamation, Proceedings, Federal Inter-Agency Sedimentation Conference, Denver, 1947. M Summary, Agricultural Conservation Program of Pro- duction and Marketing Administration, 1948. ra National Resources Planning Board, Upper Gila River Basin Report, 1940. *° U. S. Forest Service. a Department of the Interior, The Colorado River (1947), H. Doc. 419,80th Cong., 1st sess. 432 |
