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Endangered Species The Colorado Wildlife Commission has listed the Rio Grande sucker as endangered. The Division of Wildlife is coordinating a recovery plan for this species. The silvery minnow is federally listed as endangered. Historically, this species occurred throughout the Rio Grande Basin in New Mexico and Texas. It now occurs only in the Rio Grande downstream of Cochiti Dam to the headwaters of Elephant Butte Reservoir in New Mexico, about five percent of its historic range. The primary limitation to recovery is a lack of perennial river flows in the middle Rio Grande, diversion structures and water quality. Groundwater The use of groundwater in the San Luis Valley dates from the late 1800s, when small-production wells were drilled into the upper few hundred feet of the confined aquifer. Since the 1970s conjunctive use by controlled recharge of surface water through ditch diver- sions and well pumping from the shallow ("unconfined") aquifer has Compact Facts United States - Mexican Water Treaty of 1906 Guarantees the delivery of 60,000 acre-feet of water annually at the International Dam at Ciudad Juarez, except during periods of extreme drought. Elephant Butte Reservoir in New Mexico was constructed pardy to ensure the nation's ability to meet this obligation. The Rio Grande Compact provides that the allocations of water to the states shall not be increased or diminished by reason of changes in the delivery or loss of water to Mexico. Rio Grande Compact of 1938 Establishes Colorado's obligation to ensure deliveries of water at the New Mexico state line and New Mexico's obligation to assure deliveries of water at the Elephant Butte Reservoir, with some allowance for credit and debit accounts. The obligations are calculated based on a schedule of deliveries. The Compact establishes the Rio Grande Compact Commission to administer the terms of the Compact. The Commission consists of one representative from each state and a non-voting federal representative. Several tributaries to the Rio Grande are not subject to the Rio Grande Compact administraton. Amended Costilla Creek Compact of 1963 Establishes uses, allocations and administration of the waters of Costilla Creek in Colorado and New Mexico. The Compact makes apportionments and allocations among specific facilities. It is administered by the Costilla Creek Compact Commis- sion, which is composed of the water officials from Colorado and New Mexico. become the predominant method of irrigation water management, particu- larly north of the Rio Grande. acre-feet annually, of which an estimated 800,000 acre-feet is from groundwater sources. Approximately 3,500 of the 6,500 wells in the basin with greater than 50 gpm capacity are in use. Source: Colorado Department of Natural Resources and RGDSS Irrigated agriculture is by far the dominant water use in the basin, using an estimated average of 2 million Groundwater Source Unconfined Aquifer in San Luis Valley Confined Aquifer Aquifer Characteristics 40 to 100 feet thick Unconsolidated alluvial sand and gravel. Clay layers separate the aquifers. Ditch and canal leakage. Percolation of surface water from streams on alluvial fans along the periphery of the valley where the clay layers are not present. Up to 1,500 gpm on the west side of the valley. Typically 50 to 700 gpm in the central and eastern parts of the valley. Usually acceptable for potable use, although in localized areas of the central valley, nitrate, sodium, and total dissolved solids are unacceptably high. Recharge Source Yield Water Quality 500 to <4,000 feet thick and estimated to average 2,000 feet thick across most of the San Luis Valley. Upper portions same as unconfined. Sandstones and fractured volcanic rocks in deeper portions. Deep percolation of surface water from streams on alluvial fans along the periphery of the valley where the clay layers are not present. From 50 gpm to over 3,000 gpm in the southern and western parts of the valley. Typically acceptable for potable use. Source: Colorado Department of Natural Resources ( Rio Grande Basin Facts - 4 )