D6

of the discussion that follows is therefore dictated in part by the availability of those data: first, a summary of the water resource and its development in each of several areas where we have information concerning the ground water as well as the surface water; then a discussion of several regional problems that have been developed or magnified during the recent Southwest drought.

San Luis Valley is a plain about 90 miles long from north to south and 50 miles wide, and thus is nearly as large as the State of Connecticut. Although this plain is 7,500 to 8,000 feet about sea level, it is properly termed a valley because it is bordered on three sides by mountains that are 10,000 to more than 14,000 feet above sea level. The Rio Grande and other streams have had no part in forming the valley, which is a structural trough between two prongs of the Southern Rocky Mountains the San Juan Mountains to the west and the Sangre de Cristo Mountains to the east. The Rio Grande continues southward from San Luis Valley all the way to the Mexican border, in a continuation of this same structural depression. Although the Rio Grande and other streams have not formed the San Luis depression, they have gone far toward filling it, and are primarily responsible for the establishment of the present broad valley floor. The deposition of sediment by the Rio Grande in comparatively recent geologic time has formed a topographic divide in the northeastern part of San Luis Valley and has created a closed basin having a drainage area of 2,940 square miles. Because of this natural condition, several tributaries of San Luis Valley are not tributary to the Rio Grande and make no contribution to the river; indeed, the contribution is in the opposite direction, for the Rio Grande loses some water to the closed basin.

Several major physiographic provinces-the Basin and Range, Colorado Plateaus, Rocky Mountains, and Great Plains provinces, which farther north span the entire breadth of Nevada, Utah, and Colorado all converge in the vicinity of San Luis Valley (National Resources Committee, 1938, p. 198). In such a focal point of geologic activity, it might be expected that San Luis Valley would be like no other in the United States. True, San Luis Valley is quite spectacular in its present setting of rugged mountains, but if the alluvial sediments that underlie the valley floor could be removed, the result would be a gigantic trough almost 3 miles deep, with its bottom below sea level. The total volume of unconsolidated sediments in the valley is unknown in fact, the total thickness is not known at any point; however, in a recent oil test the drill penetrated more than 5,200 feet of sand, gravel, and clay before it struck a thick section of volcanic rocks, beneath which was more sand, gravel, and clay. The well was bottomed in gravel at a depth of 8,023 feet. On the basis of this well log it was estimated that ground water storage in San Luis Valley is of the order of a billion acre feet, which is more than the total estimated water production of the upper Rio Grande in three centuries.

As in many other arid basins of the Southwest, the layers of clay in the fill of San Luis Valley serve as confining beds and create artesian pressure in the underlying beds of sand and gravel. One well drilled to a depth of 1,000 feet crossed more than 50 separate flows of water. As estimated by Powell (1958) there are probably 7,500 flowing wells in the valley; in addition, scores of artesian wells are equipped with large pumps. The flowing and pumped artesian wells have a potential yield of about 500,000 acre feet a year, but many are shut in during part of the year, and the actual yield is not known. Most of the artesian wells are used for irrigation and it is believed that they supply water for the complete or supplemental irrigation of about 150,000 acres. The flow of some wells has diminished over the years, perhaps as a result of deterioration or local interference or overdevelopment. In general the artesian aquifers are regarded as not fully developed.

San Luis Valley also has a shallow unconfined aquifer which is far better than the artesian aquifers as a recorder of the effects of climatic fluctuations and of man's development of the water resources. This unconfined aquifer receives water from the artesian aquifers, both by upward leakage and by downward percolation of water drawn from artesian wells for irrigation. It receives water also by deep percolation of precipitation and especially of surface water applied for irrigation. Water is discharged from the shallow aquifer by irrigation wells, by canals and drains, and by evapotranspiration.

Most of the present shallow aquifer was not saturated until the beginning of large scale diversions of surface water for irrigation. The history of irrigation in the past 80 years includes

• (1) irrigation in the central lowlands, with resulting rise of water table until extensive areas became waterlogged;
• (2) westward migration of farming to the present areas, where, with sufficient surface water and the necessary supply and drain ditches, it is possible to maintain the water table within a few feet of the ground surface for