EFFICACY OF SHALLOW GEOPHYSICS TO SUPPORT ECOHYDROLOGY RESEARCH IN A COMPLEX GEOLOGIC TERRAIN, GREAT BASIN NATIONAL PARK, NEVADA, USA (Invited Presentation)

The Snake Creek drainage is on the eastern flank of the Snake Range in Great Basin National Park, Nevada. Located in the Basin and Range physiographic province, the study area has complex geology with a Jurassic granitic core overlain by largely Cambrian to Ordovician consolidated and weakly metamorphosed sedimentary units offset by normal and detachment faulting. Where the snowmelt-dominated creek crosses from the largely impermeable granitic core into the Paleozoic section, including the karstic Pole Canyon Limestone, the creek becomes a strongly losing reach. In 1961, this reach of Snake Creek was diverted into a 3-mile pipeline to maintain irrigation flows to agricultural lands outside of the park in the Snake Valley to the east.

An ecohydrology study is ongoing to determine the effect of this surface water diversion on the health of a cottonwood-dominated riparian zone in the largely dewatered reach below it. To supplement the surface-based portion of the study, a geophysical survey was conducted to determine the nature of the shallow subsurface below the Snake Creek drainage. Electrical resistivity imaging (ERI) was conducted at 10 locations in the dewatered and reference reaches to determine depth to bedrock and resolve the local water table. Survey lines were both cross-sectional and longitudinal. Informed by these results, eight monitoring wells were constructed and instrumented to monitor seasonal fluctuations in groundwater.

Geologic material from the boreholes was compared to the inferences made from adjacent geophysical survey lines. The geologic material helped reveal the efficacy of resistivity data in resolving geologic material properties and lithologic breaks in a complex and heterogeneous environment. The geophysical data also inform the nature of groundwater flow in both the alluvium and bedrock and the extent to which karst plays a role in inter- and intra-basin groundwater flow. These results, coupled with other ongoing research in the basin, will assist in determining the degree to which baseflow diversion has affected the riparian corridor in Snake Creek, which will have implications on how the National Park Service manages this basin in the future.