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Show Water Development Needs, Opportunities, and Programs Chaffer 3 The Rio Grande Valley is one of the oldest con- tinuously settled regions in the United States. The limited water resources have been developed and used for centuries. All water has been appropri- ated and, in some cases, overappropriated. Water use for any purpose can be expanded only at the expense of some other beneficial use. Improper use of land by overstocking and inade- quate range management has contributed to seri- ous erosion, sedimentation, and flood conditions. In turn, these conditions have seriously damaged irrigation facilities and communities. Major Fed- eral construction activity in this basin has been di- rected toward rehabilitation of these works and measures to prevent their further damage. Erosion and Sediment Control Interrelation with Flood Control, Irrigation, and Drainage Excessive rate of erosion and consequent sedimen- tation of river channels, particularly in the Pecos River and upper Rio Grande sub-basins, are vital problems. Storage reservoirs and other facilities for flood control, irrigation, and drainage projects are affected. Sediment deposits in reservoirs re- duce their storage capacity, and their useful life will be shortened unnecessarily. Additional capacity can be provided to store sediments but at consider- ably increased cost. Sediment deposits of 2,200 acre-feet, or 3,546,400 cubic yards in irrigation and drainage canals must be cleaned out periodically at considerable expense. These sediments originate on eroding range lands and in valley fills, and are cut from banks of streams (figure 5). Many river channels are steadily aggrading. This progressively reduces their ability to carry floods. In many places the river bed has been raised above adjacent valley lands, causing an ever- increasing flood threat and also causing the water table to rise and to waterlog croplands. Even where drainage canals exist or are constructed, their effectiveness is continuously reduced as the river continues to rise at the canal outlets. Often the large deltas formed by sediment in river channels above reservoirs have been occupied by dense growths of weeds, trees, and plants which, acre for acre, may consume twice as much water as do irri- gated crops. Land leveling for irrigation is costly but essential. When coarse sediment is carried onto irrigated fields, it is deposited unevenly and requires relevel- ing at intervals. When the sediment is very fine it so seals the soil surface that water cannot pene- trate properly and crop yields are thereby reduced. Nearly all flood control, irrigation, and drainage undertakings suffer from one or a combination of these damages. Sediment imposes a heavy burden on construction projects by increasing capital cost of such projects, increasing farm operating costs, in- creasing operating and maintenance costs, and materially reducing irrigated crop yield in some instances. Some units such as the Middle Rio Grande Conservancy District and the Carlsbad Irri- gation District have thus far been unable to deal satisfactorily with problems of sedimentation. Watershed Control Upstream and Related Land Programs The present situation.-Sediment is a major problem in the upper Rio Grande and Pecos River sub-basins. The McMillan Reservoir, on the Pecos River, has already lost 90 percent of its use- fulness as a storage reservoir for the Carlsbad Proj- 297 |