ANALYZING GEOLOGIC CONTROLS OF RIPARIAN MEADOW COMPLEXES USING MULTIPLE GEOPHYSICAL TECHNIQUES IN THE CENTRAL GREAT BASIN, NV

Riparian meadow systems in the Central Great Basin are of interest because they support the majority of ecosystem diversity in the region. These riparian meadows are highly dependent upon groundwater levels, thereby making them vulnerable to fluctuations. Many of these systems are actively degrading due to incision of the through-going streams, resulting in a lowered water table and modification of the associated ecosystems. This modification is usually classified by a loss of wetland meadows and invasion of several species of sagebrush. Geologic controls, such as bedrock geometry and sediment distribution, are important variables in the meadow complexes because of their control on the overall hydrology of the system. We hypothesize that sediment facies associated with side-valley alluvial fans and cross-cutting fault-related bedrock steps interact to constrict groundwater flow and thus have a dominant influence on the hydrology. Testing this hypothesis is critical in the development of a long-term management plan for maintaining and/or restoring existing meadow complexes.

Three geophysical techniques were used in conjunction to analyze the bedrock/sediment interaction involved in the formation of six “typical” riparian meadow complexes. Seismic reflection data and seismic refraction tomography data were collected to analyze bedrock structure and topography, while ground penetrating radar (GPR) data were collected to determine the stratigraphic variability. These data were combined with existing borehole data to ground-truth the geophysical data. Correlated bedrock surfaces are identifiable on both the seismic reflection and seismic refraction tomography data. The GPR data in conjunction with borehole information confirms and extends areas of alluvial fan related sediment distribution. Integration of these three geophysical techniques is useful because they provide more hydrogeologic information than could be obtained with the individual techniques.