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Show more satisfactory solution to this perplexing prob- lem is required. It must take into account normal geologic processes of wearing down the land, so as to take advantage, where possible, of such processes. For example, it may be found desirable to build low dams primarily to raise intermediate base levels of stream channels, to flatten stream gradients, and to reduce bank cutting and the carrying capacity of runoff. Such measures should be supplemented at the same time with necessary land surface controls. Reduction of Flood Damages The Present Situation Floods of the upper Rio Grande are of two types: those caused by snow melt in the mountains during the April to June period, sometimes augmented by rain, and those caused by summer rainstorms of high intensity but short duration. Snow melt is the chief source of damaging floods. Normally a large part of the New Mexico snow pack melts before the melt from the Colorado mountains reaches the densely settled valley. Occa- sionally, however, a warm spell causes rapid melt- ing throughout the upper basin. At such times, floods are destructive until the waters are caught in Elephant Butte. These snow melt floods are a source of concern to valley inhabitants because they last several weeks and maintain a constant pressure on the levee system. Damage to crops is heavy, and much bottom land is ruined by deposition or scour or even destroyed by bank cutting. Damage to physical improvements in urban centers and to roads and railroads is also heavy. Summer rainstorms are a source of flash floods. Although they ordinarily do not cover a large area, they are of great intensity and sometimes deliver relatively large volumes of water. The Sangre de Cristo Mountains are especially notorious for such storms; they average more than 60 convec- tional storms per year, a number exceeded only by the East Gulf Coast area. Intensities during these storms, some of which last less than an hour, reach a rate of from 4 to 8 inches an hour for 5-minute periods, with a total rainfall for many storms of less than 0.5 inch. Some may last several hours and deposit up to 4 inches in an hour over a rather restricted area. One storm in the east Pecos drainage is reported to have brought 9.91 inches of rain in 24 hours, and another 15 inches in 4 days. A larger part of the rainfall in most of these storms could be absorbed and tempo- rarily held in the soil were there an adequate cover. Loss of plant cover and layers of litter from the soil, however, allows more rainfall to run off with de- structive force in a short time. (See figure 4.) Flash floods from local intense storms may swell from a small trickle in a minor tributary to a raging torrent many feet deep in a matter of minutes. They are destructive not only because of their flashy character, but also because of the heavy load of sediment and debris they carry. Occasionally, peo- ple crossing streams or camping or fishing alongside them have been swept away by the walls of onrush- ing water. These flash floods usually cause damage along minor tributaries. They seldom are great enough in volume to raise the main stream to flood stage. Sediments carried by flash floods are de- posited on fields and in channels. They often block channels, causing succeeding floods to rise to still greater heights. Commonly, rainfall plays a greater part than snow melt in main stream floods of the Pecos River and lower Rio Grande sub-basins. Otherwise, the flood picture is similar. Aggradation of the main river channel progres- sively reduces its capacity to carry flood water. The river bed has been raised above adjacent lands in many places. The average rise of the bed of the Rio Grande at Albuquerque is about 4 inches a year. The flood threat is ever increasing. Average annual tangible recurrent flood damages along the main stem are estimated by the Corps of Engineers at $623,000 above El Paso and $250,000 in the Pecos Valley. Preliminary studies by the Department of Agriculture indicate that damages along tributaries may exceed those along the main stem. Communities have provided local flood protec- tion works. Wherever irrigation districts have been formed, levees have been built to protect lands from floods. Lately local interests in many instances have been unable to cope with increasing flood hazards and with growing drainage problems pre- sented by aggrading river channels. The Corps of Engineers has provided emergency repairs to the levee system of the Middle Rio Grande Conservancy District and elsewhere in the middle valley of the Rio Grande and has constructed emer- gency flood protection works at Roswell. No major reservoirs have been built for flood con- trol alone. Storage capacity of 100,000 acre-feet is provided at Caballo Reservoir on the Rio Grande for partial flood control below Elephant Butte Dam. 303 |