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Show parts per million, waters of the Rio Grande carry a much heavier burden. The dissolved solids in the Rio Grande at San Acacia Diversion Dam range from 250 to 2,380 parts per million and average 360. The tons per acre-foot of river water range from 0.30 to 3.24 and average 0.49. In a 24-hour period, the Rio Grande carries 1,230 tons of solids in solu- tion past this point. Pecos River Sub-basin From its source in the southern Sangre de Gristo Mountains to its confluence with the Rio Grande, the Pecos River flows about 926 miles. Elevations range from over 13,000 feet above sea level in the Truchas Mountains to about 1,000 feet at the mouth. From the source to the vicinity of Alamo- gordo Reservoir, the stream drains a mountainous region and the channel for the most part lies in a deep canyon. Downstream from Alamogordo Dam to Red Bluff Reservoir at the New Mexico-Texas State line the river flows through a series of valleys and canyons. Below Red Bluff Reservoir the valley widens and the stream meanders through gently rolling hills to the vicinity of Girvin, Tex. In the 180 miles below Girvin, the river's course is through a canyon which is over 300 feet deep at the mouth. The major tributaries of the Pecos River are the Rio Hondo, Dark Canyon, and Toyah Greek. Average annual precipitation ranges from about 11 inches in Texas to 35 inches in the high moun- tains of New Mexico. About 75 percent may be expected from May to October. Annual fall of snow follows the same general pattern and ranges from 2 inches near the mouth to 300 inches in the headwaters. Melting of the snow pack in the mountains may cause flooding at times if combined with rain. However, peak discharges from such floods cause damage only within a hundred miles' reach of the river's source. Floods in the reach downstream are caused by general rains over the basin and by violent thunderstorms. (See figure 4.) Pecos River and its tributaries are generally perennial streams in their headwaters, but in the lower sections of the basin the surface flow ceases as a result of evapo-transpiration, percolation into sandy beds, and irrigation diversions, or a combina- tion of these causes. The National Resources Plan- ning Board estimated that average annual water production in the Pecos sub-basin amounts to 1,095,000 acre-feet and represents only about 4 per- cent of total precipitation. Of this yield, 67 percent is in New Mexico and 33 percent in Texas. Ground water resources are of major significance in the sub-basin. Most of the irrigated area in the middle basin of the Pecos and a large part of the lower basin obtain water from wells and springs. Above Alamogordo Reservoir ground water is used only for domestic and stock water purposes. In the Roswell Artesian Basin ground water with- drawals total about 350,000 acre-feet per year. Extensive use of ground water has depleted the flow of the Pecos River. New Mexico now controls further appropriations of its ground water. Salinity problems in the Pecos drainage are even more serious than those in the Rio Grande. For example, waters of the Pecos sampled at Alamo- gordo Dam average 1,630 parts per million, carry- ing 2.22 tons of solids per acre-foot of water. The condition of the waters of the Pecos worsens down- stream; as measured at Acme, Artesia, Carlsbad, Malaga, Red Bluff, and Orla, Tex., the dissolved solids have averaged 5,430 parts per million, carry- ing 7.3 tons of solids per acre-foot of water. The dissolved solids carried past Orla average about 3,580 tons per day. These waters are unsatisfac- tory for irrigation and for many other purposes. Water quality, therefore, is a serious problem in water use. One of the causes of high salinity, especially of ground waters, is the presence of salt beds of various kinds just beneath a porous limestone. After many years of discussions and negotiations, an agreement has been reached by New Mexico and Texas relative to apportionment and delivery of waters of the Pecos River. The Pecos River Com- pact was approved in 1948. 295 |
