San Luis Valley Regional Habitat Conservation Plan Draft for Public Review
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2.3Water Resources and AdministrationSurface WaterThe headwaters of the Rio Grande, the Conejos River, the Alamosa River, and other drainages begin as small streams in the San Juan Mountains, flowing east to the Valley floor where they join the mainstem Rio Grande as it flows south into New Mexico. In addition to the natural streams and rivers, more than 600 miles of mapped ditches and canals in the Valley have been developed over the last century to support irrigated agriculture (Figure 6). While these diversions have altered the location and extent of native riparian habitat, and have resulted in a historic loss of wetlands in the Valley, the current irrigation infrastructure now provides surface and ground water flows that help sustain the current mosaic of riparian habitat. For example, the development of an extensive network of canals and irrigation agriculture has created irrigation-induced wetlands where none previously existed (CNHP 2004). Ground WaterThe subsurface geology of the Rio Grande Basin, which lies beneath the Valley, is a series of saturated sedimentary and volcanic rock layers. These layers comprise aquifers from which numerous wells draw water. Water continues to recharge the aquifers by percolation from surface streams, leakage through canals, and recharge from the mountains surrounding the Valley. This complex aquifer system includes a shallow unconfined aquifer and a series of deeper, confined aquifers that are interconnected and hydrologically connected with the surface water system. The unconfined aquifer is the uppermost water-saturated layer of sand and gravel, down to a depth of about 100 feet across most of the Valley. Below the unconfined aquifer are a number of clay layers underlain by deeper water-bearing layers of sand, gravel, and fractured volcanic rocks that make up the confined aquifer. Water flows naturally from some wells drilled into the confined aquifer due to natural artesian pressure. One unique feature of the Valley is the Closed Basin. This large area in the northern part of the Valley drains about 2,900 square miles and is separated from the rest of the Valley, at least in part, by a hydraulic divide; thus, the surface streams in the Closed Basin are not directly tributary to the rest of the Rio Grande Basin, and much of the water that flows into the basin is lost through evapotranspiration and evaporation. The allocation of waters in the Rio Grande Basin is governed by the State system of water rights administration that has been in place for more than a century, and additionally by the Rio Grande Compact on the Rio Grande and Conejos River. Rio Grande CompactThe Rio Grande Compact is a formal interstate agreement between Colorado, New Mexico, and Texas that dictates the amount of water that must pass annually from Colorado to downstream States on the Rio Grande. The Compact was ratified by all three States in 1938 and enacted as a Federal statute by Congress in 1939. Under the Compact, Colorado has an annual obligation to deliver water on a percentage basis, roughly 24 to 60 percent of the flow in the Rio Grande and 0 to 70 percent of the flow in the Conejos River system. The Colorado Division of Water Resources is responsible for administering the Rio Grande and Conejos River systems to ensure Colorado’s compliance with the Compact (Vandiver 2005). An inherent premise of the Compact is that the river channels between the Valley in Colorado and El Paso, Texas will remain free flowing and without major obstruction to carry water throughout the reaches of the Rio Grande Basin system (Id.). If water channels were allowed to deteriorate, Colorado would have great difficulty meeting its Compact obligations, which would subject the State to litigation from New Mexico and Texas. This would destroy the interstate comity that the Compact provides and would subject Colorado and its water users to great financial and legal liability, as well as losses of water supply. State System of Water Rights AdministrationWater rights administration in the Valley is based on Colorado’s Doctrine of Prior Appropriation where the available water is allocated and delivered to the calling priorities. This system of “first in time, first in right” has been in place for more than 100 years. The surface streams in the Valley are overappropriated and there is no opportunity for new appropriations because the Compact and decreed water rights in place have legal entitlement to all of the water in the system. In practice, the State Engineer typically curtails surface water rights and supplements these water deliveries with Closed Basin Project water to satisfy Colorado’s Compact delivery obligations. Because of the overappropriated nature of water in the Valley, the Colorado Division of Water Resources, the State, and the Division Engineer require flexibility to administer and deliver water throughout the year to meet the simultaneous needs of decreed water right users and Colorado’s Compact obligations. Closed Basin ProjectThe Closed Basin Division of the San Luis Valley Project was authorized by Congress on October 20, 1972. In 1980, the Bureau of Reclamation began construction on the Closed Basin Project. Designed to pump water out of the Closed Basin, the Project discharges water from numerous wells in the unconfined aquifer into a conveyance channel that delivers water to the Rio Grande. The total ultimate production capacity of the Closed Basin Project was estimated to be about 117,000 acre-feet per year. However, actual production to date has averaged 24,000 acre-feet per year. This low average is partly due to production curtailment in years when the water was not needed to help satisfy Compact deliveries, and the biofouling of many of the wells in the Project. Basin water users agree that a more realistic goal would be a production rate of 65,000 acre-feet per year. By law, the Closed Basin Project deliveries are categorized into three types of uses:
The District currently shares duties with Reclamation in administering and maintaining the Closed Basin Project and its features. The District holds the water rights for the Closed Basin Project. Figure 6. Water Resources Platoro Dam and ReservoirPlatoro Dam and Reservoir, located high on the Conejos River, is another feature of the San Luis Valley Project that was built to control floodwater, and provide supplemental water to irrigate about 73,890 acres of land in the Conejos Water Conservancy District (CWCD), which is 40 miles away from Platoro Dam and Reservoir and within the HCP plan area. Platoro Dam and Reservoir was completed in 1951. The dam was built and owned by Reclamation, and for many years was operated and maintained by the CWCD. In 1989, Congress authorized the transfer of the reservoir to the CWCD. In 1991, the responsibilities for operation and maintenance of all of the facilities and regulation of joint use space (conservation and flood control) below 10,027.57 feet were transferred from Reclamation to the CWCD (Reclamation 2005).14 Pursuant to an agreement with Reclamation, CWCD paid for its share of the construction costs, and assumed the maintenance and operation of the dam and reservoir. Because this reservoir was built after the Compact was signed, it is referred to as a “post-compact reservoir,” and is subject to special restrictions under the Compact. The U.S. Army Corps of Engineers retains exclusive authority over Platoro Reservoir for flood control purposes and directs the CWCD in the operation of Platoro Reservoir for such purposes. Other Significant ReservoirsBesides Platoro Reservoir, which is described above, other significant reservoirs in the HCP plan area include Sanchez Reservoir (south of San Luis), La Jara Reservoir (west of La Jara), and Smith Reservoir (south of Blanca). |