GEOMORPHIC EFFECT OF GROUNDWATER FLUX ON THE CHANNELIZED LOOSAHATCHIE RIVER, SHELBY COUNTY, TENNESSEE

A study was designed to determine the geomorphic effect of groundwater fluid flux on channelized rivers through a multi-method approach. The Loosahatchie River in eastern Tennessee, was chosen as the study area since it traverses several major aquifer outcrops of the Mississippi Embayment. Theoretical modeling by Urbano et al. (2006) indicates that rivers crossing these transitions will undergo a major, systematic, change in groundwater/surface water exchange. To determine the localized effects of groundwater flux, two reaches were defined and partitioned into grids. Flux locations were located by using the groundwater temperature gradient and detected by means of type K thermocouple probes inserted to depths of 10 and 20 cm below the river bed as well as a third probe located 1 cm above the riverbed surface. River bed elevations and structures were then recorded using aerial photography and GPS. Each reach was reexamined on a weekly basis to determine the progression of features. Foci of groundwater discharge located by temperature probe were found to predominantly occur along geologic unit contacts as well as along the river thalweg. Temperature profiles obtained immediately above the bed surface were also found to closely match those obtained by the subsurface probes. Discharge foci temperatures obtained 20 cm below river bed surfaces were found to range from 2 to 7 degrees Celsius lower than river water indicating varying rate of flux. Locations of groundwater flux also control the form of migrating sand bars by limiting the amount of sediment deposited in spring locations due to increased pore pressure. Evidence suggests that shallow geology of channel sediments dictates location of groundwater flux foci, and that these foci control the thalweg location, not low-flow river flow dynamics.