You are here:
Patterns of Ground Water Movement in a Portion of the Willamette River Floodplain, Oregon
Citation:
FAULKNER, BART, K. J. FORSHAY, AND RENEEJ BROOKS. Patterns of Ground Water Movement in a Portion of the Willamette River Floodplain, Oregon. Presented at the 2009 American Water Resources Association Annual Conference, Seattle, WA, November 09 - 12, 2009.
Impact/Purpose:
To develop channel reconstruction and restoration projects.
Description:
In reaches unconstrained by revetments, the Willamette River and its floodplain along its lowland mainstem is a continually evolving system. Several channel reconstruction and restoration projects have been implemented or planned in order to obtain beneficial services along the river system. In many cases, the aquatic habitat improvements expected from these efforts are strongly dependent upon flows between ground water and surface water, and the ground water component of these flows (hyporheic flow) is often more difficult to measure, quantify, and generalize in a geomorphic framework. In cooperation with the McKenzie River Trust, the U.S. EPA Office of Research and Development installed and instrumented 50 monitoring wells along a portion of the Willamette near its confluence with the McKenzie River. This is a region of high activity for restoration and channel reconstruction projects, and the McKenzie River Trust is implementing restoration projects on Green Island, near Coburg, Oregon, where the wells were installed. The area is representative of the Willamette system in its natural geomorphic condition. Instrumentation includes continuous monitoring of water elevation and temperature in the wells. Water level changes are significantly correlated with river stage, even in wells up to 1 km from the river, and preliminary stable isotope analyses indicate extensive hyporheic flow. Mapping of the potentiometric surface indicates ground water mounding during wet season in older forested floodplains, and the greatest potential for hyporheic flow in the younger bar sediments. These results have implications for nitrate retention and temperature buffering. Slug tests were conducted in 41 wells to characterize the hydraulic conductivity. Study of historical aerial photographs showed the highest conductivities (up to 492 m/day) occur in remnant stream channels and young gravel bars, and the lowest (down to 18 m/day) occur in the interior, coalesced locations of the main island. Results of calibrating a ground water flow model (MODFLOW) are emerging as data is collected. For further information on this presentation, please contact the U.S. Environmental Protection Agency, National Risk Management Research Laboratory, 26 W. Martin L. King Dr., Mail Stop: G75, Cincinnati, OH 45268
