OCR Text |
Show Incidental flood control is gained from reservoirs operated for irrigation projects. The International Boundary and Water Com- mission has canalized the river and constructed local bank protection works along the lower Rio Grande. Facilities under Construction and Authorized Two flood control projects are now under con- struction by the Corps of Engineers. One of these, the Jemez Canyon Dam on Jemez Creek, N. Mex., provides 120,000 acre-feet of storage; 30,000 acre- feet are allotted to flood control, and 90,000 acre- feet to trapping of sediments. The other is a chan- nel improvement project on Willow Creek to protect Creede, Colo. The International Boundary and Water Commis- sion has under way a program of levees and channel improvement between Fort Quitman and the Gulf. Several projects primarily for flood control have been authorized but are not yet under construction by the Corps of Engineers. These projects, all of which are In New Mexico, include Chamita Dam on Rio Chama, Rio Grande Floodway, and Blue- water Floodway on Bluewater Creek. In Colorado, the San Luis Project, authorized for construction by the Bureau of Reclamation, will provide substan- tial flood control storage. Situation at End of Present Program The projects described are only an initial phase in flood control. The authorized flood control works for the upper Rio Grande Basin are a start toward an Integrated flood control plan. They do not provide flood control storage on the main stem to protect the middle Rio Grande Valley from the large snow-melt floods originating in the Colorado mountains. The Corps of Engineers considers such control an essential part of the over-all program. Studies have not yet progressed sufficiently to determine what structures, if any, will be required in the Pecos River sub-basin before flood protec- tion will meet present needs. Preliminary reports by the Department of Agriculture indicate that the more frequent flash floods of minor tributaries in this area could be controlled by an adequate pro- gram of watershed management although major floods would be reduced by only a small amount. Completion of the program of dam construction and levee and channel improvement in the lower 304 Rio Grande Basin carried out by the International Boundary and Water Commission is counted on to provide adequate flood protection to affected areas. Program for Future Construction The Corps of Engineers considers that additional construction will be required to supplement the au- thorized projects to protect existing improvements. Additional projects contemplated for the upper Rio Grande Basin include four dams to provide 1,416,000 acre-feet of storage. One of these, at the Chiflo or a suitable alternative site, is on the main stem of the Rio Grande and is considered essential to control floods in the Cochiti-Elephant Butte area. Local flood protection works for Alamosa, Colorado, are also needed. A possible program in the Pecos sub-basin in- cludes four other dams for combined irrigation stor- age and flood control. A fifth would provide stor- age exclusively for flood control. Local protection for four areas in the valley is contemplated. Principles of Control and Management of Flood- waters Control of floodwaters requires both watershed treatment and main stream control works. Improvement of the vegetal cover on range and forest lands, which constitute most of the catch- ment area of the Rio Grande Basin, will increase the infiltration of rain water into the soil, retard overland flow, and thereby reduce immediate sur- face runoff and soil erosion. On the basis of plot studies by the Department of Agriculture, water- shed management can be expected to increase infiltration rates by at least one-third. A complete watershed treatment program, including minor structures to retard further surface runoff, can be counted on to prevent an important part of the floodwater damages caused every summer by flash floods in arroyos and minor tributaries. Because of high erosional potentials in the Rio Grande watershed, revegetation of land surface and minor works are generally inadequate to check the headward cutting of gullies. To reduce the carry- ing power of flash runoff from frequent torrential rains calls for a new type of engineering, which may be called, for want of a better term, physiographic engineering. In this case it would call for raising intermediate base levels of stream erosion at stra- tegic locations in channels. These base-level con- |