7.1 SAN LUIS VALLEY & RIO GRANDE BASIN
The Colorado portion of the Rio Grande Basin, located in south-central Colorado, encompasses approximately 7,500 square miles and constitutes Colorado Water Division 3. The administration of water in the Rio Grande Basin is governed not only by Colorado state law, but also by requirements of the Rio Grande Compact with New Mexico and Texas. The Rio Grande and Conejos River originate in the eastern San Juan Mountains and are the dominant watersheds in the basin. These rivers flow through the San Luis Valley, which is an open, almost treeless, intermontane basin bounded by the foothills of the San Juan Mountains on the west and the Sangre de Cristo Range on the east. The 3,200 square mile San Luis Valley encompasses the counties of Saguache, Rio Grande, Alamosa, Conejos, and Costilla. The San Luis Valley represents a significant ground water resource that is recognized nationally. The overall Rio Grande watershed also includes Mineral and Hinsdale counties, as well as a small area in the southeastern corner of Archuleta County (Figure 7.1-1).
Total population in the San Luis Valley is about 45,000 and the population in the entire Rio Grande Basin is only slightly higher, as few people reside in the mountainous areas. The San Luis Valley is an open, relatively treeless, flat valley floor with some hills in the southern end, whereas the remaining part of the Rio Grande watershed, the mountainous areas outside the central valley, are generally heavily vegetated, rugged, and steep. Irrigation is widely practiced in the valley, with the major crops being potatoes, barley, vegetables, and alfalfa.
The basin-fill deposits that form the aquifer system in the San Luis Valley are hydraulically interconnected with the alluvium of the Rio Grande and its tributaries within the valley. Due to this hydraulic interconnectivity, the Rio Grande alluvial aquifer within the valley and the San Luis Valley basin-fill aquifers are discussed together in this subchapter. The San Luis Valley is part of the Rio Grande Rift, a north-trending series of interconnected, down-faulted, and rotated blocks, known as grabens. The formation of these grabens has accommodated as much as 30,000 feet of basin-fill deposits in the San Luis Valley. A generalized geologic cross-section depicting the stratigraphic and structural relationships in the San Luis Valley is presented as Figure 7.1-2.