Geophysical Imaging of Near Surface Hydrostratigraphy in Arid Ephemeral Stream Systems Near Yuma, AZ

Wednesday, 17 December 2014
Dennis Lee Harry1, Nicholas A Sutfin1, Jeremy R. Shaw2, Joshua Faulconer3, Andrew J Genco1, Ellen Wohl1, Stephanie K Kampf3 and David J. Cooper2, (1)Colorado State University, Department of Geosciences, Fort Collins, CO, United States, (2)Colorado State University, Department of Forest and Rangeland Stewardship, Fort Collins, CO, United States, (3)Colorado State University, Department of Ecosystem Science and Sustainability, Fort Collins, CO, United States
Ground penetrating radar and DC electrical resistivity profiles image the upper 4 m of the subsurface beneath ephemeral streams in Yuma and Mohave Washes in the Sonoran Desert, 30 km northeast of Yuma, Arizona. The geophysical data are tied to trenches to establish a lithostratigraphic interpretation. Archie’s Law, calibrated to resistivity measurements on soil samples from each site, is used to estimate in-situ soil pore saturation. Three different stream types are surveyed. Increasing in stream order, these are incised bedrock with alluvium fill, incised alluvium, and braided streams. Three radar facies are identified on the basis of reflection amplitude, continuity, and dip. Near the surface, RF1 (0.5-1.5 m thick) contains laterally continuous sheetlike deposits interpreted to be active channel gravel, sand, and cobble deposits reworked during floods. Below, RF2 contains moderately continuous downlapping and onlapping reflections interpreted to be partially lithified Pleistocene gravel and cobble valley fill deposits. The underlying facies RF3 is nearly reflection free, but at the larger washes contains weak reflection similar in character to RF2. In the smaller washes, RF3 contains abundant diffractions. Two electrofacies are identified. The shallowest, EF1, extends from the surface to ~2.5 m deep. EF1 encompasses radar facies RF1 and RF2, with resistivity ranging from 250-1500 ohm-m. Estimated soil moisture in this facies ranges from 2-40%, and varies up to 20% laterally over 2-5 m distances in the smallest washes. Facies EF2 coincides with radar facies RF3, with resistivity ranging from 10-300 ohm-m and estimated pore saturation is estimated to exceed 70%. Electrofacies EF1 is inferred to represent a relatively dry surficial layer that includes the modern channel deposits and the upper ~1 m of the Pleistocene strata. At the larger washes, EF2 is interpreted to be Pleistocene valley fill, distinguishable from the overlying lithologically equivalent Pleistocene deposits by lower resistivity resulting from higher moisture content. Near surface lateral soil moisture variations in the smaller washes is strongly influenced by the subsurface stratigraphic architecture (facies RF2). In the larger washes, soil moisture variations are more subtle, changing gradationally with depth and downstream.