On December 10 Dr. Laura Toran of Temple University presented to the ESF community novel methods for monitoring river restoration. The challenge in river restoration is seeing below the riverbed and monitoring how restoration influences the exchange of river water with groundwater. Ecosystem health relies on this exchange, and scientists have given the name ‘hyporheic zone’ to the ecotone below the riverbed where the mixing is most pronounced.  Dr. Toran uses a geophysical monitoring technique called electrical resistivity tomography to see below sand and gravel along the riverbed-water interface and measure how much surface water is mixing in the hyporheic zone. The technique works when a low resistivity solute, such as salt, is injected in the river and electrical resistivity probes are inserted into or on-top of the river bed. Before the solute is injected the probes take a reading of the riverbed resistivity, which is considered background. After the solute is injected the probes measure resistivity at 10 to 15 min cycles, depending on the probe cable length and number of probes, and departure from background is noted. Additional work is required to invert the signal and create an image of the below riverbed porous media. What Dr. Toran discovered was river restoration structures such as j-hooks used in Natural Channel Design (NCD) create areas upstream of the j-hook where river water lingers longer in the hyporheic zone.  A j-hook structure on Onondaga Creek south of Syracuse, NY is shown below.

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We worked with Dr. Adam Ward of University of Iowa using a similar electrical resistivity technique to examine hyporheic exchange around a cross-vane, a NCD river structure similar to the j-hook.  ESF student Jesse Robinson, SU student Maggie Zimmer, Dr. Ward’s students, and I were able to use Dr. Ward’s innovative tools to compare how river water mixing with groundwater at a cross-vane is to the mixing at a riffle, which helps restoration engineers improve their cross-vane designs when their goal is to better mimic riffles.

Background image of river cross-sections, noting 0% departure in color bar. Top image is at the cross-vane, middle image is at the natural riffle, and bottom image is the time-series of the salt solute breakthrough curve.

Background image of river cross-sections, noting 0% departure in color bar. Top image is at the cross-vane, middle image is at the natural riffle, and bottom image is the time-series of the salt solute breakthrough curve.

Lower resistivity at the cross-sections when the salt solute has now mixed into the hyporheic zone. These images are of the riverbed, and show the larger extent of mixing in the cross-vane as compared with the natural riffle.

Lower resistivity at the cross-sections when the salt solute has now mixed into the hyporheic zone. These images are of the riverbed, and show the larger extent of mixing in the cross-vane as compared with the natural riffle.

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