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Cooling Along Hyporheic Pathlines in a Large River Riparian Zone
Citation:
Faulkner, B., K. Forshay, R. Brooks, R. Adeuya, AND S. Cline. Cooling Along Hyporheic Pathlines in a Large River Riparian Zone. Presented at American Geophysical Union's 45th Annual Fall Meeting, December 03 - 07, 2012.
Impact/Purpose:
Floodplains can contribute to hyporheic cooling and moderation of temperature for rivers, but extent and magnitude are dependent on ground water hydrology. Here we illustrate the controls and dynamics of hyporheic cooling in the ground water of a large river floodplain with field data and a hydrologic modeling approach in a region where cooling may influence the formation of coldwater refugia, a valuable ecosystem service for the preservation of salmon habitat.
Description:
Floodplains can contribute to hyporheic cooling and moderation of temperature for rivers, but extent and magnitude are dependent on ground water hydrology. Here we illustrate the controls and dynamics of hyporheic cooling in the ground water of a large river floodplain with field data and a hydrologic modeling approach in a region where cooling may influence the formation of coldwater refugia, a valuable ecosystem service for the preservation of salmon habitat. Hyporheic flow and ground water temperature were extensively monitored and characterized along a floodplain section of the Willamette River, Oregon, USA. Numerical flow modeling was done for the strongly contrasting dry and wet season flow patterns. During the warm dry season of our study, we observed significant temperature reduction in ground water with distance along most of the pathlines. Examination of particle tracking results indicate that in our site’s floodplain areas, hyporheic flow during the wet season often has a locally downward component due to recharge from infiltration of rainfall, although dry season ground water flow also often moves deeper along the longer pathlines. In the dry season, pathlines trend to horizontal. Overall, the observed dry season temperature profiles obeyed the analytical solution to a one-dimensional steady-state governing equation for heat transfer in porous media, with differing thermal Péclet numbers. In one gravel bar, a mean pathline length of about 600 m reduced temperature 18 to 11 degrees Celsius. This cooling occurs over a residence time of 3-4 years before re-emergence, and incorporates the water table fluctuation, and wet season recharge from infiltration of rainfall, that produces a locally downward trend of the hyporheic water prior to moving upward and discharging to cutoffs and alcoves. In some of the islands, wet season ground water underwent sufficient mounding to halt or reverse dry season river water that had entered the hyporheic zone. I
