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GROUNDWATER-SURFACE WATER EXCHANGE AND IMPLICATIONS FOR LARGE RIVER RESTORATION
Fernald, A. G., D H. Landers, AND P. J. WIGINGTON JR. GROUNDWATER-SURFACE WATER EXCHANGE AND IMPLICATIONS FOR LARGE RIVER RESTORATION. Presented at 8th International Symposium on the Ecology of Regulated Streams, Toulouse, France, July 17-21, 2000.
Movement of river water into and out of high-porosity alluvial deposits can have an important influence on surface water quality and aquatic habitat. In our study of a 60-km reach of the Willamette River in Oregon, USA, we: 1) used tracers to estimate the rate of exchange between river and groundwater; 2) measured water quality in river source water, hyporheic flow, and receiving side-channel surface water; and 3) estimated historic and current hyporheic flow rates along the river.
Tracer studies revealed high rates of surface water-groundwater exchange in reworked deposits on channel reaches that were unconstrained by geologic or anthropogenic bank-hardening features. At six bar deposits instrumented with wells for detailed study of hyporheic flow, river source water entered the gravel deposits and followed hyporheic flow paths to emerge in side-channel surface water. We found that compared to river source water, hyporheic dissolved oxygen, temperature, and ammonium decreased, while hyporheic specific conductance, soluble reactive phosphorus, and nitrate increased. Emerging hyporheic flow affected receiving surface water characteristics, with the magnitude of this effect dependent on site substrate porosity and hyporheic flow rate. Analysis of historic and present-day channel features shows that there has been a large loss of channel complexity and actively reworked deposits.
Restoration strategies of removing bank-hardening structures and delivering elevated peak flows would promote channel movement and formation of high-porosity alluvial deposits. This, in turn, would increase total hyporheic flow, which appears to promote water quality stability overall while contributing to aquatic habitat diversity at specific sites.