Geology and water resources of Las Vegas, Pahrump, and Indian Spring Valleys, Clark and Nye Counties, Nevada

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Geology and water resources of Las Vegas, Pahrump, and Indian Spring Valleys, Clark and Nye Counties, Nevada
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Report on the geology and groundwater of southern Nevada.
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Accompanying map has been removed.
ABSTRACT Las Vegas, Pahrump, and Indian Spring Valleys are situated part of Las Vegas Valley and the city of Las Vegas, are the main transportation routes to Los Angeles, California, about 300 miles south, and to Salt Lake City, Utah, about 450 miles north of Las in Clark and Nye Counties in southwestern Nevada. The city of Las Vegas, in the south-central part of Las Vegas Valley, is the chief commercial center for the three valleys. The Union Pacific Railroad and U. S. Highway 91, which pass through the southern Vegas. The population of Las Vegas Valley is chiefly dependent for its livelihood upon a resort and tourist trade, a limited chemical and mining industry, and the railroad. The people of Pahrump Valley are chiefly farmers and ranchers and the few people in Indian Spring Valley depend for their livelihood upon the tourist trade and the operation of one large ranch. The climate of the area is arid, for the average annual precipitation in the valleys is less than 10 inches, and there are no perennial surface streams. The water supply for the valleys is obtained from springs and wells, except at the town of Henderson in Las Vegas Valley, where a pumping station and pipe line supply water from Lake Mead. A rapid increase in population in Las Vegas Valley, beginning in 1941, caused an apparently critical water shortage there, and in Pahrump Valley increased agricultural development resulted in further exploitation of ground-water supplies. The purpose of the study upon which this report is based was to determine the occurrence, source, and amount of ground water available in the three valleys. The three valleys lie near the southwestern boundary of the Great Basin. They are bounded by high, rugged mountain masses with precipitous slopes which abut against relatively gently sloping alluvial aprons. The highest and largest mountains are the Spring Mountain and Sheep Ranges. The alluvial aprons usually terminate at their lower ends in playas. Remnants of the alluvial aprons extend far up the mountain canyons and, in many places, blanket the mountain slopes to elevations as high as 9,000 feet. In part of Pliocene and Pleistocene time, during and immediately following deposition of the sediments of the aprons, the mountains bounding the three valleys were probably buried deeply in alluvial materials which have since been partially removed by erosion. The alluvial slopes are being eroded at the present time, although in some places they are sites of deposition. Sediments are being deposited in the lower parts of all the valleys. The mountains are everywhere being eroded. Drainage in Pahrump and Indian Spring Valleys is interior, to playas that occupy the lowest portion of each valley. In effect, drainage in most of Las Vegas Valley is likewise interior, although if appreciable surface runoff occurred the water would drain to the Colorado River through Las Vegas Wash in the extreme southeastern part of the valley. The rocks exposed in the area range in age from pre-Cambrian to Recent. Generally the older rocks of pre-Cambrian, Paleozoic, Mesozoic, and early Tertiary age form the mountains, and the rocks of Miocene (?), Pliocene, Pleistocene, and Recent age form the relatively unconsolidated materials within the valleys. Of the older rocks only the Sultan limestone of late (?) Devonian age and the Monte Cristo limestone of early and middle Mississippian age are important water-bearing formations, and usually they occur above the regional ground-water level. The other older rocks are relatively impermeable and are not important aquifers. They impede ground-water movement and act as barriers to form the boundaries of the ground-water reservoirs. The Esmeralda (?) formation of late Miocene (?) age and the Muddy Creek formation of Pliocene (?) age are thick deposits of chiefly fine-grained alluvial materials with a few thin sand and gravel lenses. They crop out in five widely separated localities in Las Vegas Valley and probably are present in the valley fill beneath the younger sediments in three valleys. These beds are not important as aquifers at the present time. Deeper drilling in the valleys may produce wells of moderate yield in the sand and gravel lenses in the sediments of the Esmeralda (?) and Muddy Creek formations. However, water from them may be highly mineralized. The upper 700 to 1,000 feet of sediments in the valleys are the older alluvial deposits of gravel, sand, silt, and clay, chiefly of Pliocene (?) and Pleistocene (?) age. They are probably underlain by the Muddy Creek and Esmeralda (?) formations, and in some places they are overlain by a thin veneer of Recent playa and eolian sediments. These Pliocene (?) and Pleistocene (?) alluvial deposits form the alluvial apron and are typical alluvial-fan deposits. The upper part of the alluvial apron consists chiefly of gravel and sand beds, some of which grade into silt and clay toward the lower parts of the valley; others extend persistently toward the axes of the valleys and interfinger with the silt and clay beds. These persistent gravel layers are believed to represent periods when the streams had relatively great carrying power, probably periods of more humid climate. The silt and clay beds are inferred to represent periods when the streams had smaller carrying power, during times of aridity. The alluvial-fan materials are generally coarser and the deposits are much thicker and topographically higher in the valleys opposite the larger canyons in the mountains. In the valleys opposite the smaller canyons and along the mountain slopes, they consist chiefly of fine materials and are thinner and topographically lower. Numerous logs of the alluvial materials have been recorded from wells drilled in the southern part of Las Vegas Valley and in the central part of Pahrump Valley. They show that clay, sandy and silty clay, and caliche make up by far the largest part of the valley deposits near the lower ends of the alluvial fans. Layers of gravel and sand ranging from 1 to 20 feet in thickness occur infrequently there. The logs also show that these layers of gravel and sand are lenticular and thin rapidly toward the central parts of the valleys. Probably most of the gravel and sand lenses are limited in horizontal extent and are more or less imperfectly interconnected. Most of the ground water used in the three valleys is obtained from wells and springs and is supplied by the gravel and sand lenses of the valley fill. In the Las Vegas Valley more than three-fourths of the wells draw water from aquifers ranging from 250 to 450 feet below land surface, designated as the Shallow Zone of aquifers. This zone is separated from the underlying Middle Zone of aquifers, which range from 500 to 700 feet in depth, by a persistent 10- to 50-foot-thick blue clay layer. Several wells of large yield draw water from aquifers in the Middle Zone. A few wells drilled to depths of more than 700 feet have encountered thin water-bearing beds as deep as 1,225 feet. All the waterbearing beds below 700 feet are included in the Deep Zone of aquifers. In Pahrump Valley confined water is encountered in wells at depths ranging from 165 feet to more than 900 feet. In Indian Spring Valley confined water has been found at depths ranging from 400 to 600 feet. Ground water also occurs in the three valleys at shallow depths (100 feet or less). In parts of the valleys this water is under slight artesian pressure, in other parts of the valleys it occurs under water-table conditions. This water is referred to in this report as the "near-surface" water. Playa and lacustrine deposits of Pleistocene age occur in the lower parts of the three valleys. These beds consist of superficial deposits of relatively impermeable silt and clay which are rarely thicker than 50 feet. The playa, eolian, and wash deposits of Recent age consist chiefly of unconsolidated gravel, sand, silt, and clay. The deposits are usually less than 100 feet thick. They are only locally significant as aquifers. In the vicinity of Indian Springs and in the southeast part of Las Vegas Valley water, used chiefly for domestic purposes, is withdrawn from occasional thin gravel and sand lenses occurring in Recent deposits. Outstanding geologic structural events include block faulting, which occurred previous to late Mesozoic time, and overthrusting and folding during Mesozoic and during early Tertiary and Quaternary time. Minor faulting and folding were probably synchronous with and related to both the overthrusting and the block faulting. Evidence that major faults and other large-scale structural activities displaced the older alluvial deposits was not observed anywhere in the three valleys. Small normal faults of probable Recent and late Pleistocene age were observed in the older alluvial deposits and in the Muddy Creek formation in Las Vegas Valley. These faults are probably a result of differential compaction in the younger relatively unconsolidated sediments, and probably do not cut the older bedrock, as do faults of Recent age in adjacent regions. Movements of ground water in Las Vegas Valley are significantly affected by these faults. They act as partial barriers that impede the movement of water through the various aquifers. Moderately permeable beds in the valley fill were probably offset against less permeable beds, thus partly or wholly damming the flow of water through the permeable beds. Some of the ground water thus impeded moves upward along the fault zones and issues as springs near the traces of the faults. The location and origin of Kyle, Stevens, and Las Vegas Springs near the foot of the fault scarps in Las Vegas Valley are apparently a result of such faulting. The older structures in the indurated bedrock of the mountains also affect the movement of ground water. Most fault zones are cemented and generally form ground-water dams. Where the attitude and permeability of the rock strata are favorable, the water is brought to the surface as springs. When joints occur in soluble formations, they generally transmit large quantities of water. The only source of ground water for the three valleys is precipitation on the higher areas of the Spring and Sheep Mountains. However, only a small part of the precipitation recharges the alluvial-fan and valley-fill materials that compose the groundwater reservoirs. The rest of the water from precipitation on the area is lost by evaporation and transpiration. The water that reaches the ground-water reservoirs is ultimately discharged through springs and wells and by evaporation and transpiration. Estimates based on the available precipitation data, and checked with information from all available geologic and hydrologic data, show that the annual recharge of the ground-water reservoir in Las Vegas Valley is between 30,000 and 35,000 acre-feet. The total annual discharge from the ground-water reservoirs in Las Vegas Valley probably never exceeded 35,000 acre-feet until 1946. Water levels have declined in the valley. They may be expected to continue to decline until the cones of depression in the piezometric or pressure-indicating surface, caused by withdrawal of water from wells and springs, have grown sufficiently to intercept the amount of recharge necessary to balance the total withdrawals of ground water. Locally, much of the excessive decline of water levels in Las Vegas Valley has been a result of local overdevelopment caused by close spacing and heavy pumping of wells. However, the available data indicate that ground water probably is now being pumped from storage; that is, more water is being taken from the reservoirs than is entering them from the recharge areas, and that therefore part of the water-level decline has resulted from overpumping. Thus, continued withdrawal of substantially more than 35,000 acre-feet of ground water annually will result in continued, and possibly increasing, decline of the water level and in overdevelopment of the ground-water supply in Las Vegas Valley. Of the total discharge of ground water in Las Vegas Valley probably 5,000 to 8,000 acre-feet, or 12 to 15 percent, is lost by evaporation and transpiration. Also, it is estimated that possibly 15 percent of the total discharge is wasted through lack of conservation, mostly within the city of Las Vegas. It appears that at least half the water thus lost can be utilized by further development of wells in the near-surface reservoir and by increased, more efficient conservation of supplies now obtained from the Shallow, Middle, and Deep Zones of aquifers. In Pahrump Valley approximately 23,000 acre-feet of water is annually available for recharge, and about 17,000 acre-feet is annually discharged from wells and springs. Water levels have declined during the short period of record and they may be expected to decline until the cones of depression have grown sufficiently to intercept the amount of recharge necessary to balance the total withdrawals of ground water. However, some ground water is available for additional development in Pahrump Valley. Although sufficient data are not available to show whether there is a substantial unused supply in Indian Spring Valley, it appears that some additional ground water is available there also. The chemical character of the ground water in Las Vegas Valley differs considerably from place to place. In general the quality is better in the vicinity of the city of Las Vegas than it is toward the lowest part of the valley to the south. The ground water in the vicinity of the city of Las Vegas is suitable and is used for both domestic and irrigation purposes. However, in the south part of the valley the water is not suitable for either domestic or irrigation use. In Pahrump Valley the best water is found along the east side and poorer water in the central part. Although the water in the central part of the valley has a higher concentration of dissolved solids than that from the east side, it is suitable for domestic use and safe for irrigation.
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Water resources bulletin, no. 5
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