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Water-Resources Investigations Report 01-4220

Hydrogeologic Framework of Antelope Valley and Bedell Flat, Washoe County, West-Central Nevada


Recent population growth in west-central Nevada has led to the expansion of urban development to rural valleys north of the Reno-Sparks metropolitan area (fig. 1). Currently, the Truckee River provides more than 75 percent of the water supply for the Reno-Sparks area. For most of the outlying valleys to the north, however, ground water is the only source of water, particularly where Truckee River water may be impractical to deliver. The Washoe County Department of Water Resources (WCDWR) has initiated several hydrologic investigations in these valleys, focusing primarily on describing the ground-water flow systems in basin-fill aquifers that underlie the valleys and on determining the potential effects on these systems of regional ground-water development.

The first step toward evaluating regional ground-water flow is to develop an understanding of the regional hydrogeologic framework and its control on ground-water movement. A hydrogeologic framework often can be inferred from information about the geometry and thickness of basin-fill deposits. In most valleys, however, the thickness of these deposits frequently is unknown. Geophysical methods can provide a practical and economical means to estimate the thickness of basin fill and to describe a valley's general hydrogeologic framework.

In 2000, the U.S. Geological Survey (USGS), in cooperation with the WCDWR, began a 2-year water-resources investigation to add to knowledge of regional ground-water flow systems in west-central Nevada and to provide background information for future hydrologic studies. This investigation uses surface geophysical methods to assist in describing the hydrogeologic framework of the Antelope Valley and Bedell Flat Hydrographic Areas 1 (fig. 1).

Located in southern Washoe County about 15 mi north of Reno, the Antelope Valley and Bedell Flat Hydrographic Areas have a combined area of 68 mi2. Antelope Valley, the smaller (17 mi2) of the two areas, is topographically closed. There is limited use of ground water in Antelope Valley, mainly from domestic wells associated with large-parcel development. The 51 mi2 Bedell Flat is mostly undeveloped, but urbanization is expected to increase ground-water use in the next 5 to 10 years.

Antelope Valley is bounded on the east by Hungry Mountain and Warm Springs Mountain (which are nearly 6,000 ft in altitude) and on the west by Freds Mountain (nearly 7,200 ft in altitude) and by the northern extension of an unnamed mountain (pl. 1). Forming the southern boundary are the northern extensions of two unnamed mountains that separate Antelope Valley from Lemmon Valley. A series of low hills forms the northern boundary, which coincides with part of the southern boundary of Bedell Flat. The floor of Antelope Valley is about 2 mi wide and 5 mi long and covers about 55 percent of the total drainage area. In the lowest part of the valley is a playa at an altitude of about 5,100 ft.

Dogskin Mountain, the highest mountain in the study area at nearly 7,500 ft in altitude, makes up most of the northern and eastern boundaries of Bedell Flat. A series of low hills and a narrow alluvial divide complete the eastern boundary. Bedell Flat is bounded on the west by the Sand Hills, which have altitudes approaching 6,200 ft. A low, broad alluvial divide between the southern extension of the Sand Hills and the east flank of Freds Mountain makes up most of the southern boundary. The valley floor, which decreases in altitude from about 5,200 ft in the southwest to about 4,800 ft in the northeast, covers about 58 percent of the drainage area. The drainage line on the valley floor in Bedell Flat trends to the northwest and is estimated to discharge about 70 acre-ft of streamflow annually through a narrow draw to Red Rock Valley (Rush and Glancy, 1967, p. 30).

The principal source of precipitation to the region is the Pacific Ocean, which typically provides moisture from October to June. During the summer months, tropical air from the Gulf of California produces scattered but intense convective showers (Houghton, 1969, p. 5; Brenner, 1974). In the study area, annual precipitation on the valley floors is about 8 in./yr; the surrounding mountains receive about 15 in./yr (Berger and others, 1997, p. 31). Estimated annual precipitation is 9,000 acre-ft/yr in Antelope Valley and 28,000 acre-ft/yr in Bedell Flat (Rush and Glancy, 1967, p. 21). Ground-water levels, reported by well drillers, generally are between 80 to 100 ft beneath the floor of Antelope Valley. According to a limited number of well drillers' logs in Bedell Flat, ground-water levels are about 100 to 200 ft beneath the valley floor in the southern part of the valley and less than 5 ft beneath the northwestern part.

Purpose and Scope

The purpose of this report is to describe the hydrogeologic framework of Antelope Valley and Bedell Flat. This framework is defined by (1) the rocks and deposits that transmit ground water or impede its movement, and (2) the combined thickness of basin-fill deposits and volcanic rocks. For purposes of this study, Quaternary and Tertiary basin-fill deposits and Tertiary volcanic rocks are collectively defined as Cenozoic deposits. Plutonic and metamorphic rocks are defined as pre-Cenozoic basement.

Geophysical methods were used to infer the form of structural features and to estimate the thickness of Cenozoic deposits in Antelope Valley and Bedell Flat. Gravimetric data were compiled from 351 gravity stations. Data from 14 of these stations are publicly available on a CD-ROM (Ponce, 1997); data from 126 stations were collected by Thomas C. Carpenter (Thomas C. Carpenter, Consulting Geophysicist, written commun., 1998), and data from 211 stations were collected in April 2000 as part of this study (Jewel and others, 2000). Seismic-refraction data were collected along five profiles during several weeks from August to December 2000. The seismic-refraction data were used as additional control points for gravity interpretation in areas of limited borehole data.

1 Formal hydrographic areas in Nevada were delineated systematically in the late 1960's by the U.S. Geological Survey and Nevada Division of Water Resources for scientific and administrative purposes (Cardinalli and others, 1968; Rush, 1968). The official hydrographic-area names, numbers, and geographic boundaries continue to be used in Geological Survey scientific reports and Division of Water Resources administrative activities.

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