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031770 CHAPTER 2 WATER RESOURCES Surface Water The total watershed area of the San Luis Valley is about 5 million acre:i. "'''ithin this area, mere are approximateiy 516,000 acres of BLM land or about 10 percent of the total. These lands are not important from a water production standpoint. Average annual runoff from BLM lands has been estimated at 35,000 acre-feet (Gifford et. al., 1975) or about .85 inch. This contrasts with water yields of over 30 inches from the high altitude headwater areas. There are 56 perennial streams within the planning area, none of which originate on BLM land. The combined length of these streams totals about 630 miles, of which 73.5 miles pass through BLM land. Drainages originating on BLM lands are either ephemeral or intermittent; most are ephemeral, Runoff is usually the result of intense summer thunderstorms. In heavy snow years, however, spring snowmelt can produce significant runoff. Flow in these channels is reduced by heavy transmission losses, primarily by percolation into the ground water system. Surface runoff from these drainages rarely reaches perennial streams. Ground Water The floor of the San Luis Valley is underlain by water- bearing sedimentary deposits that are miles thick. Two major aquifer systems are present. Unconfined (water table) ground water is present at shallow depths practically anywhere on the valley Door. This aquifer is underlain by a huge, complex, confined (artesian) aquifer system. The confined aquifer produces large quantities of good quality water. Since leakage occurs between the two systems, development of either aquifer has an effect on the other. The occurrence of ground water is much more variable around the fringes of the valley Door. Along the base of the Sangre de Cristo Mountains, ground water is abundant in alluvial and colluvial deposits and is also in sedimentary, igneous, and metamorphic rocks. The western side of the valley is surrounded by volcanic formations and water can occur in these rocks and in underlying igneous and alluvial formations. Most wel1s located above the valley Door yield relatively small quantities of good quality water. Approx- imately 100 springs have been located on BLM lands, most of which are concentrated in the northern end of the valley. BLM withdraws less than 7,000 acre-feet of ground water (including the Blanca Wildlife Habitat Area). This is less than I percent of the total 750,000 acre-feet annual withdrawal, Water Quality All of the perennial streams passing through BLM lands have good to excellent quality water. The exception is Kerber Creek, which passes through about one-half mile of BLM land and is heavily polluted from mining wastes in the privately owned Bonanza Mining District. The quality of ground water is generally very good. The exception, however, is the unconfined aquifer of the valley Door. In many locations, this aquifer contains high levels of dissolved solids. GEOLOGY,TOPOGRAPHY,AND MINERALS MANAGEMENT ~ Geology and Topography The San Luis Valley (SL V) is part of the much larger Rio Grande Rift Zone, which extends from southern New Mexico northward through the San Luis and Upper Arkansas Valleys to its northern termination near Leadville, Colorado. This intermountain valley opens southward towards New Mexico and is approximately 150 miles long and 50 miles wide. The SL V is bordered on the east by the linear Sangre de Cristo Mountains, the result of extensive block faulting during the Laramide Orogeny. This faulting has resulted in the placement of Precambrian basement, Paleozoic sedimentary, and Tertiary intrusive rocks in contact with Tertiary valley- fill deposits. The western side of the SL V is flanked by the San Juan Mountains, the result of extensive Tertiary volcanism. In sharp contrast with the steeply faulted eastern side of the valley Door, the Oligocene volcanic rocks of the San Juans gently dip eastward into the valley floor where they are interbedded with valley-fill deposits. The subsurface of the valley itself is broken by two major horst blocks that essentially bisect the basin from Saguache, Colorado, sonthward to the New Mexico border. The southernmost horst is the result of block faulting, whicb has brought Oligocene volcanic rocks to the surface forming the San Luis Hills. Extending north from this structure is the easterly tilted, deeply buried Alamosa Horst composed of Precambrian age rocks. On either side of this horst are two deep basins; the Baca Graben to the east and the Monte Vista Graben to the west. &timated depths to Precambrian 2-8