OCR Text |
Show Grande sub-basin above Fort Quitman is consumed in it. The valley of the river in this sub-basin is under- lain by unconsolidated sediments. Ground water is pumped for supplemental irrigation in the San Luis Valley. All cities in the area, with the excep- tion of Santa Fe and, in small part, El Paso, obtain their water supply from wells. Depletion of ground water has caused El Paso to purchase irrigated farms with water rights. The ground water of the valley is tributary to the river. Its extensive use will eventually result in lower flows of the Rio Grande. The waters of the Upper Rio Grande are appor- tioned among Colorado, New Mexico, and Texas in accordance with the Rio Grande Compact which became effective in 1939. This compact defines the rights of each State to the waters and accounts for stream flow and water use by each State in ac- cordance with those rights. Definite schedules, which vary with the annual water yield, are estab- lished for delivery of annual volumes of water at key points in the Rio Grande Basin, and divergence from those schedules is accounted for as "debits" or "credits." Debits arise from failure to deliver the scheduled supply of water by any State and impose certain restrictions on that State's use of water. Credits for any State are created by delivery of water at key points in excess of that required by schedules and confer on that State further rights for use of water. Equitable distribution of the waters of the Rio Grande above Fort Quitman, Tex., is provided for by the treaty of 1906.2 The United States agreed to deliver to Mexico 60,000 acre-feet annually, ex- cept in case of extraordinary drought or serious ac- cident to the irrigation system in the United States. To insure delivery of the water and to supply irri- gated lands in the Rio Grande Project, Elephant Butte Reservoir was constructed near Truth or Consequences, formerly Hot Springs, N. Mex. More recently, the American Diversion Dam was constructed near El Paso to regulate delivery of water to Mexico. Shallow ground waters on both sides of the river channel above Elephant Butte Reservoir are con- centrated by evaporation so as to become toxic to crops and vegetation. Salinity problems in the basin are increasing in intensity. 2 Convention of May 21, 1906 (34 Stat. 2953). Lower Rio Grande Sub-basin Below Fort Quitman, Tex., the Rio Grande flows through dry mountainous country with small iso- lated valleys as it passes through the Big Bend. Below the Big Bend section it drains a roughly broken terrain with narrow valleys and broad high tablelands. The Rio Conchas entering from the Mexican side, and the Pecos and Devils Rivers from the United States are major tributaries. From its juncture with the Pecos the Rio Grande crosses the Balcones Escarpment and heads southeastward through hill country. Approaching the delta region it picks up flows of the Mexican rivers, Salada, Alamo, and San Juan. These are the largest tributaries of the Rio Grande. The largest valley in this reach is approximately 70 miles long and, in the United States, about 30 miles wide. It ex- tends from the head of the delta to the gulf and is known locally as the Magic Valley. The flow of the Rio Grande normally is low at Fort Quitman because of depletions above that point for irrigation and other uses. Below, how- ever, it picks up the discharges from tributary streams, mostly Mexican, and reaches Rio Grande City, near the head of the delta area, with an aver- age annual volume of about 5,300,000 acre-feet. Irrigation in the delta region, both in the United States and Mexico, makes heavy demands on the river and there are many days when the entire flow is utilized. However, an average of 3,750,000 acre-feet of this volume is wasted in the Gulf of Mexico each year because of the erratic nature of the runoff in the lower basin. Ground water resources of the subbasin are con- fined principally to the Devils River and delta, al- though scattered ranches, farms, and towns, par- ticularly along the outer rim of the basin, obtain domestic supply from wells. The 1944 Treaty between the United States and Mexico allotted the waters of the Rio Grande from Fort Quitman to the Gulf of Mexico to the respec- tive nations 3 with certain assurances against deple- tion by Mexico below a stipulated quantity. The allotment at Falcon Dam is approximately 60 per- cent to the United States and 40 percent to Mexico. The International Boundary and Water Commis- sion administers the treaty provisions. Unlike rivers of the humid East, where dissolved solids in stream flow may be between 60 and 100 "Treaty of February 3, 1944, between United States and Mexico. Treaty Series 994, 49 Stat. 1219. 293 |