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Rio Grande Basin Colorado w UJ ~ E 8 Legend l Elevation Relief ~ Lakes and Reservoirs Rivers and Canals Roads UUater district Boundaries 1d 0 10 20 30 Miles r .~ - - .* ~.~~'~ ~ ~ _ .~ .~y~~~ s San L ,is Valley y~ '. s." ~ I ~+reat Sand dunes. ,~~ - -i ~- ~ at'I Monument el N orlte ~I _ Ofl€ 'yy a •' ~. '~' _. Oft#~ VISiB r '"' ~' ~. i - ~j ~' i ~ - ~~AL~B fnOSB ,• .,. ~ , `_ _ # i$~~„Wa"! ~ ~+~~~ r X71 ~~ e } 1'" y'~ ~ - ~ . - v ~" m Figure 1-1 Rio Grande Basin (Full Size, from RGDSS Feasibility Study) Of the 7,500 square miles, there is an area of approximately 3,000 square miles in the northern part of the valley that does not naturally drain into the Rio Grande because it is separated by a low divide in the shallow ground water table formed by the alluvial fan of the Rio Grande on the west and alluvial material from the Sangre de Cristo Mountains on the east. This non-tributary area is known as the Closed Basin. The Rio Grande main stem rises in the San Juan Mountains in the vicinity of Creede, Colorado and flows easterly where it is joined by the South Fork of the Rio Grande at South Fork, Colorado. The main stem continues east to Del Norte where it flows out onto the San Luis Valley floor in a southeasterly direction. The main stem continues this path until it reaches Alamosa, where it abruptly turns south and flows to the state line. 2.2 Hydrogeologic Setting of the San Luis Valley The San Luis Valley is considered to be a high mountain desert with cool summers and cold winters. Most of the precipitation on the valley floor comes in the form of scattered summer afternoon rain showers. Annual average precipitation ranges from approximately 7 inches at Alamosa to approximately 45 inches at Wolf Creek Pass. However, most streamflow originates from snowmelt in the San Juan and Sangre de Cristo mountains. Previous authors and investigations have offered water budgets for the valley. These estimates have historically been contested and were therefore not incorporated into this study. The San Luis Valley is part of the Rio Grande depression, anorth-trending series of intermontane basins extending from Texas to central Colorado. Despite its apparent simplicity and lack of topographic relief, the subsurface geology of the valley is complex and not yet fully understood. In general terms, the valley is called a graben, a geologic term meaning adown-dropped block of the earth's crust, bounded on the west side by the San Juan Mountains and on the east side by a major fault at the edge of the Sangre de