U.S. Geological Survey U.S. Geological Survey Hyporheic flow and nitrogen cycling were evaluated for a 15-square-kilometer area adjacent to the Willamette River, a ninth order river in Oregon. Potentiometric surface maps over an annual cycle demonstrated the presence of highly dynamic hydraulic gradients between the river and adjacent aquifer. At the river stretch scale (tens of kilometers), measured gains and losses of up to 5 to 10 percent of streamflow were consistent with fluxes implied by potentiometric surface maps and apparently reflect regional ground water/surface water interactions. At the river reach scale (1 to 2 kilometers), measured gains and losses on the order of 5 percent of streamflow were interpreted as representing primarily local hyporheic exchange, probably occurring in near-river gravels.
Hyporheic exchange also was demonstrated with isotopic and chemical data collected from the shallow hyporheic zone. The origin of sampled hyporheic zone water ranged from a mixture composed mostly of regional ground water to essentially 100 percent river water. The common assumption that ground and river water mix primarily in the river channel is not applicable in this system.
Isotopic and chemical data indicated that nearly complete vegetative nitrate uptake and/or nitrate reduction occurred in portions of the hyporheic zone. Investigation using microbial enzyme assays at one hyporheic site demonstrated the existence of a strong vertical redox gradient, with nitrate-limited denitrification potential in deeper sediment and both significant nitrification and denitrification potential in shallower sediment. Thus, nitrogen cycling in the hyporheic zone of this large-river system likely is affected by dynamics of ground water/surface water interactions that control fluxes of nitrogen and other redox species.