HYDROGEOLOGY OF PERENNIAL HOT AND COLD SPRINGS IN BLACK CANYON BELOW HOOVER DAM, NEVADA AND ARIZONA

Springs and seeps discharging into the Black Canyon (BC) of the Colorado River downstream from Hoover Dam are important hydrologic features of the Lake Mead National Recreation Area that support recreational activities as well as riparian ecosystems native to the Mojave Desert. Increases in water use from the urbanization of southern Nevada and northwestern Arizona may potentially impact spring discharge. The U.S. Geological Survey is assessing hydrologic conditions at select springs in BC in cooperation with the National Park Service. The study will evaluate hydrologic sources and flow paths resulting in discharge of hot and cold springs in close proximity. Geologic mapping and kinematic analysis of structures associated with BC springs will be used to evaluate structural controls on ground-water flow.

Most springs and seeps in the BC are located within 10 km of Hoover Dam and discharge from Tertiary intrusive and volcanic rocks and Precambrian gneiss. Discharge is restricted to elevations between the surface of Lake Mead and the Colorado River representing approximately 150 m of hydraulic head. Water likely circulates through major faults, but then diffuses near the surface allowing it to emanate from broad areas on canyon walls. This flow pattern makes accurate temperature and discharge measurements difficult. Temperatures between 12 to 55°C have been reported; however, cooler water may be caused by surface flow. Stream gages are being installed to monitor flow variations in different reaches of several tributaries.

Previously reported chemical and stable-isotope compositions of BC springs have been interpreted as evidence for a hydraulic connection between the thermal springs and water impounded in Lake Mead. However, strontium isotope data from both hot and cold springs have a limited range of ⁸⁷Sr/⁸⁶Sr values (0.71138 to 0.71177 for 9 sites) with a median value (0.71153) that is distinct from Lake Mead water (0.71014). U-series dating of relatively thin (several cm) encrustations of gypsum and efflorescent salts at some discharge areas will be used to evaluate whether discharge preceded impoundment of Lake Mead and whether the U and Sr isotopic compositions of the water have changed as a consequence.