Hydrogeology of Selected Perennial Hot and Cold Springs in Black Canyon below Hoover Dam, Nevada and Arizona

The Springs below Hoover Dam in the Black Canyon of the Colorado River are important hydrologic features of the Lake Mead National Recreation Area. Not only are these springs utilized for recreation, they are unique aquatic, riparian, and phreatophytic ecosystems that support sensitive aquatic species endemic to the Mojave Desert. Rapid population growth in southern Nevada and increased development in Detrital Valley, Arizona, following completion of the Hoover Dam by-pass, will lead to increased demand for water supplies in the area, including ground-water withdrawal. It is unclear how sustained pumping of ground water from these areas may affect aquatic and riparian ecosystems in Black Canyon.

Objectives

The objectives of the proposed study are to: (1) document existing hydrologic conditions at select springs in the Black Canyon area and compare current spring discharge with records of discharge prior to impoundment of Lake Mead; (2) examine the hydrogeologic conditions that result in the discharge of hot and cold springs in close proximity to one another in Black Canyon; and (3) delineate potential source areas and flow paths of water discharging at the hot and cold springs in Black Canyon.

Strategy and Approach

Geochemical, hydrologic, and geologic data will be collected to help characterize the ground-water system in the Black Canyon area. Major element, trace element, stable oxygen isotopes (δ¹⁸O), deuterium (δD), strontium-87/86 (⁸⁷Sr/⁸⁶Sr), uranium-234/238 (²³⁴U/²³⁸U), and nutrient concentrations will be determined on water and rock samples from springs in Black Canyon and springs and wells in nearby areas to document seasonal or longer-term variability in water chemistry, to characterize water quality conditions, and to delineate ground-water sources. Because of the possible hydraulic connection between water impounded in Lake Mead and discharge at Black Canyon springs, records of paleodischarge from tufa or siliceous sinter mounds at several springs will be dated using U-series or radiocarbon methods and analyzed for δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, and ²³⁴U/²³⁸U. New and existing discharge, water-level, and water-chemistry data collected by land-management agencies, other entities, and this study will be compiled, reconciled, reviewed for quality assurance/quality control purposes, and entered into the publicly accessible USGS National Water Information System. Geologic mapping and kinematic analysis of structures associated with the co-located, hot and cold springs in Black Canyon will be used to evaluate the geologic controls on flow paths. Five geologic cross-sections will be constructed, including sections roughly east-west from Eldorado to Detrital Valley and NW-SE, north-south, and NE-SW from Las Vegas Valley and Muddy Mountains to Black Canyon.

Relevance and Benefits

Evaluating the quantity, quality, and source of springs within Black Canyon below Hoover Dam will provide water-resource information that can be used by National Park Service scientists and natural-resource managers, as well as representatives of other government entities and local water development interests. Results from this study will help the USGS meet its mission of providing reliable, impartial information useful for water management in a high-growth region and will directly address the Environment & Natural Resources portion of the USGS strategic plan.

Map showing springs and wells in the study area. Inset shows the principal study area and a detailed view of springs in Black Canyon.