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in Colorado and nine closed basins in New Mexico. The runoff in Colorado is derived mainly from snowmelt; conse- quently, streamflows are b1gh in spring and early summer. The head- waters of the Rio Grande are relatively pure and the main dissolved constituent is calcium bicarbonate. Downstream natural inflow and return flows from irrigated areas increase the salinity. The annual runoff in the upstream portion of New Mexico is derived from snowmelt and rainstorms. As the Rio Grande flows southward, it becomes progressively more saline. This is due not only to return flows from irrigation, but also to inflow from the tributaries includ- ing the Jemez River, Rio Puerco, and Rio Salado, which carry dissolved sulphates of calcium, sodium, and magnesium. The tributaries north of the Jemez are all of low salinity. Excessive erosion and consequent sedimentation of river channels is a major problem in the Rio Grande in New Mexico above San Marcial. In many places, the riverbed is higher than adjacent lands. Historically, 60 percent of the sediment production is derived from the Jemez River, Rio Puerco, and Rio Salado. There are several small transmountain diversions importing water from the Upper Colorado Region into the Colorado portion of the Rio Grande which average about 3,6 MGD (4,032 AF!AN). In addition, the San Juan-Chama transmountain diversion project will eventually divert an average of about 98.2 MGD (110,000 acre-feet annually) from the San Juan Basin of the Upper Colorado to the Rio Grande in New Mexico. During the first 6 years of operation, about 90,000 acre-feet annually were diverted to the Rio Grande from the San Juan Basin. The Lower Rio Grande, including the Pecos River, and other drain- ages, and the Rio Grande in Texas produces natural runoff primarily from flood flows produced by high-intensity rainfall. Small but im- portant contributions to runoff occur from springs, but snowmelt is negligibl~ except in New Mexico's high areas. '0..::: '..,'. The Pecos River rises in the Sangre de Cristo Mountains, flows through eastern New Mexico into southwest Texas and joins the Rio Grande near Shumla. The Pecos is highly mineralized, because great quantities of evaporites and limestone are within reach of circulat- ing groundwaters; however, its sediment production is considerably less than the Rio Grande's. The flow of the Pecos is highly mineral- ized from Red Bluff Reservoir downstream nearly to the Rio Grande, at which point flows from springs greatly improve its quality. Be- cause of high salinity, the quality of Rio Grande flows is poor from Fort Quitman to the mouth of the Rio Conchos near Presidio. r :- 3 . '1-i' '- ',~ .. v -'-f 7