Scientific Investigations Report 2006–5145
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5145
Ground water flowing from major springs along the eastern margin of Death Valley currently supplies most of the water consumed locally, while also sustaining much of the local habitat supporting the unique flora and fauna of Death Valley National Park (fig. 1). These spring complexes constitute the terminus for the largest part of the Death Valley Regional Ground-Water Flow System (DVRFS; Harrill and others, 1988) that includes much of the western part of southern Nevada. Centrally located within the flow system is the Nevada Test Site, which historically has been used for testing nuclear devices; and Yucca Mountain, which recently has been selected as the single location for storing high-level nuclear waste generated throughout the United States. Past attempts to quantify the amount of ground water discharging from these valley-margin springs have been limited, and typically have focused only on a single spring complex. A more thorough and consistent quantification of the discharge from these local water sources is necessary to establish a sound basis for water rights and a baseline for assessing and documenting future changes in ground-water discharge in the park.
Of the many regional springs that exist along the eastern margin of the Death Valley (fig. 1), the Grapevine Springs complex is the least exploited for water supply and consequently contains the largest area of undisturbed spring-fed riparian habitat within the park boundary. Miller (1977) gives the only available estimate of ground-water discharge from Grapevine Springs in a reconnaissance study. This estimate is based on a limited number of springflow measurements and a cursory estimate of evapotranspiration (ET), and it is considered highly uncertain. A more accurate and reliable estimate of ground-water discharge is necessary for documenting current water needs and developing a better understanding of the long-term sustainability of this sensitive ecosystem. The U.S. Geological Survey (USGS), in cooperation with the National Park Service, began a 3-year study in 2000 to better estimate mean annual ground-water discharge from the Grapevine Springs area and the amount of ground water being transpired by local vegetation at Grapevine Springs and the other major spring complexes along the eastern margin of Death Valley.
The purpose of this report is to describe the procedures used in the study to develop estimates of (1) ground-water discharge for the Grapevine Springs area and (2) the amount of ground water being evaporated and transpired by local riparian vegetation at the major spring-discharge areas along the eastern margin of Death Valley (fig. 1). These procedures utilize high-resolution, multi-spectral, satellite imagery, and, as a consequence, have generated a variety of digital images for the valley-margin spring-discharge areas that are available from the USGS by accessing the National Spatial Data Infrastructure (NSDI) at URL: <http://nsdi.usgs.gov/>. Additionally, this report provides baseline data from which to make comparisons for assessing future changes in ground-water discharge within the park boundary.
The report presents estimates of mean annual ground-water discharge developed from estimates of evaporation and transpiration by the local riparian vegetation. Discharge estimates are not inclusive of any water diverted for human consumption or for operational support by the National Park Service (NPS) and by other private concerns in the park.
The climate of the Death Valley area is the most arid in North America. The valley lies primarily in the Mojave Desert, an area characterized by short mild winters, long hot summers, and low annual rainfall and humidity. The wide range in altitude and latitude across the area contributes to climatic conditions that vary dramatically on both seasonal and daily time scales. Temperatures range from winter lows below freezing in the mountains at higher altitudes to summer highs that exceed 120°F on the valley floor. The daily temperature range commonly exceeds 30°F. Precipitation amount, although generally small, varies considerably across Death Valley and depends on altitude, latitude, and location relative to surrounding mountain peaks. Mean annual precipitation ranges from less than 2 in. on the valley floor to more than 10 in. at higher altitudes in the Panamint Range and the Grapevine Mountains (fig. 1; Daly and others, 1994).
Death Valley is the terminal drainage for the DVRFS. Drainage features in Death Valley primarily consist of intermittent streams fed by spring snowmelt or infrequent, major storms. Only a few short reaches, located directly downgradient of major springs, flow year-round. These perennial flows increase in winter, when cooler temperatures and more stressed vegetation result in reduced rates of ET. The two primary drainage features in Death Valley are the Amargosa River in the south and Salt Creek in the north (fig. 1). The Amargosa River, the largest drainage in the region, drains about 5,800 mi² and is the only major drainage feature that originates outside the valley (fig. 1).
Death Valley lies within the southern Great Basin region, an internally drained part of the Basin and Range Physiographic Province. The region is characterized by low rainfall, intermittent streams, large internal surface drainages, and occasional spring-fed oases. The dominant physiographic features are north-trending mountain ranges separating broad, elongated valleys, which formed in response to a long and ongoing period of crustal extension (Stewart, 1980, p. 110); and elevated plateaus, which formed during a period of intense volcanism between about 15 and 8 millions of years ago. Crustal extension has resulted in large vertical displacements along north-trending faults that offset bedrock blocks creating similarly trending mountain ranges separated by alluvial- and fluvial-deposit filled valleys.
Death Valley is situated along the southeastern edge of the Great Basin region. The Panamint Range bounds central Death Valley on the west (fig. 1). This range is one of the highest ranges in the southern Great Basin and rises nearly 12,000 ft above the valley floor to an altitude of about 11,050 ft above NAVD 88 at Telescope Peak (fig. 1). The lowest point in Death Valley, 282 ft below NAVD 88, is in the central part of the valley and is the lowest point on the North American continent.
The major mountain ranges in the southern Great Basin region are composed primarily of pre-Cenozoic rocks of diverse age and lithology (Stewart, 1980). Paleozoic carbonate rock, Paleozoic and Proterozoic siliciclastic rock, and Tertiary volcanic rock constitute the primary rock types of the hills, ridges, and mountain ranges. Intermontane valleys are filled primarily with alluvium and colluvium eroded from surrounding carbonate and volcanic highlands, and lacustrine and palustrine deposits.
The water-transmitting properties of the rocks and the many geologic structures associated with the tectonic history of the area are important controls on the movement of ground water. In general, fractured carbonate rock (limestone and dolomite), fractured volcanic rock (welded tuff and lava flows), and alluvium (sand and gravel) form the major aquifers; whereas the siliciclastic rock (siltstone and quartzite), non-fractured volcanic rock (bedded tuff) and alluvium (silt and clay) form the primary confining units in the region. Faults generally are barriers to ground-water flow but occasionally can act as conduits (Winograd and Thordarson, 1975).
On the basis of rock properties and other geologic and hydrologic information, the Great Basin was divided into 22 regional ground-water flow systems (Harrill and others, 1988). Death Valley is the terminal discharge area for a large (about 16,000 mi²) flow system in the southern part of the Great Basin. Ground water in the flow system generally flows through fractured rock and coarser grained sand and gravel deposits southward and westward away from the major recharge areas centered in local and adjacent highlands toward Death Valley. Much of this water exits the flow system prior to reaching Death Valley at intermediate points of discharge through regional springs and seeps (Winograd and Thordarson, 1975). These springs and seeps typically occur in areas where major fault systems intersect the regional aquifers. Based on flow patterns, D’Agnese and others (1997) identified three primary subsystems within the DVRFS—a northern subsystem that ultimately discharges to springs and seeps at Grapevine Springs and Staininger Spring; a central subsystem that ultimately discharges to Nevares, Texas, and Travertine Springs; and a southern subsystem that ultimately discharges to Saratoga Springs (fig. 1).
Water emerging from mountain, valley-margin, and valley-floor springs supports a large diversity of plants, fish, and local wildlife in the Death Valley area. The highest discharges emerge from springs associated with high-angle faults occurring along the valley margins. These higher discharge areas often include multiple springs and seeps which, as a group, discharge anywhere from a few hundred to a few thousand gallons per minute. Fault-associated springs usually produce warm water with temperatures ranging from about 80 to 100°F. Valley-floor and mountain springs typically discharge less water at cooler temperatures, usually no more than a few gallons per minute at temperatures less than 80°F. Saratoga Springs in the southern part of Death Valley is the only large-volume spring on the floor of the valley.
Water discharging from valley-margin springs and seeps infiltrates into the surrounding soils and supports local wetlands around discharge points and riparian vegetation along drainages. Water not evaporated or transpired by the local vegetation flows downward to the water table and laterally toward the valley floor. The riparian vegetation, especially that growing near higher discharge valley-margin springs, provides habitat for numerous species of endemic and rare fish, aquatic insects, and plants, and is dominated by phreatophytes common to desert washes and wetlands, such as desert willow, honey mesquite, desert baccharis, yerba mansa, saltgrass, and desert wild grape. The water emerging from these valley-margin springs is of potable quality and supports less salt-tolerant vegetation than found on the valley floor. Typical vegetation communities associated with these springs are dense to moderately dense, and because of the steeper topography, usually are restricted to areas adjacent to discharge orifices and in narrow drainages. In less steep areas, such as on ledges and pediments, moisture extends outward at shallow depths to support more broad meadows that often are dominated by salt-tolerant species such as saltgrass. Beyond these moist areas, xerophytes dominate the typical landscape. The vegetation in these xerophyte communities is not reliant on spring discharge and includes sparse covers of creosote bush, saltbush, and desert holly.
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