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Show Watershed Management Watershed management in the Rio Grande Basin must take into account the slopes, especially in the upper sub-basin and the Pecos sub-basin, which have a high erosional potential. The tendency is for any damage to vegetal cover to start gullying at the head of old alluvial and detritus filled arroyos. This cutting lowers base levels of all drainage ways into the arroyo, which in turn lowers water tables, reduces water detention capacities of detrital fills, and accelerates cutting in the areas tributary to the new levels. Once such cutting starts, vegetation along the sides is undermined and does little to reduce erosion. A successful program of watershed management within the basin calls for (a) conservation of the productive capacity of the soil resources, (b) aid in the control and regulation of the water resources through water flow retardation, and (c) soil stabil- ization measures. These would help extend the useful life of downstream engineering works. It is essential that this program of watershed manage- ment be accelerated to alleviate the extensive land erosion and other aspects of soil deterioration. An adequate vegetal cover must be established on de- nuded areas which now contribute materially to flood and sediment damages. In addition, the pro- gram will increase recreational benefits and en- hance fish and wildlife resources. Sediment Control Soils of woodlands and grasslands are highly erodible when deprived of their natural grass cover. Heavy thunder showers of localized high intensity occur in rainy seasons and yield flash runoff. A sensitive ba-lance between vegetative control and geologic erosion is easily tipped to acceleration of normal rates. Sediment production from valley fills is far greater than from any other source in the basin. A sound program for sediment control should provide for maximum practicable retention of sedi- ment in the upstream areas and major structures to trap sediments as immediate protection of irrigated lands below. Release of clear water from reservoirs will cause deepening of the Rio Grande channel. Such degrading of the channel would facilitate some drainage of irrigated lands, lower flood heights, and result in reduction of channel losses of water. Irrigation The program for maximum use of the water re- sources should design irrigation facilities eco- nomically and consistently with other water uses in a balanced program to establish and maintain a sound economy throughout the basin. Full use of the water supply available for irriga- tion use calls for rehabilitation of existing irrigation facilities and for better management of water by water users. Irrigation systems are in need of im- provements such as better location of distribution canals, installation of adequate canal structures, im- proved drainage, and lining of ditches to reduce the loss of water and measures to salvage waters now wasted. In some cases storage reservoirs will be needed to make efficient use of waters. Importation of water from other basins and re- duction of water losses may permit expansion of irrigable acreage. Drainage Most irrigated areas in this basin have drainage problems which are intensified where the river is aggrading above reservoirs. Adequate drainage should be provided by a pro- gram of rehabilitation and extension of drainage canals in conjunction with control of sediments and channel rectification. Salvage of water would be helped by drainage works. Flood Control An effective program for flood control should include reservoirs at strategic locations to reduce flood peaks to safe channel capacities, supplemented by channel rectification and levee systems to pro- tect critical flood damage areas. The proposed program of dams and levees should be given full consideration. Local authorities should be called on to zone certain valley areas to eliminate unwar- ranted exposure of people and property to flood hazards. All of these measures should be supple- mented by improved watershed management, in- cluding erosion and sediment control in upland areas. Domestic and Industrial Water Supply All water in surface streams and all known ground water supplies are now fully appropriated. 346 |