Permeable, coarse-grained, alluvial deposits occur within the channel and flood plain of the main stem Willamette River and many of its tributaries, particularly those draining the Cascade Range. Within these permeable deposits, stream and ground water may exchange within an area known as the hyporheic (hy-po-ree -ic) zone--a diffuse and somewhat ill-defined region that forms the boundary between the stream channel and the adjacent ground water flow system. Water in the hyporheic zone flows along the overall downvalley direction of the stream channel, but fluctuates between the channel and the subsurface (Bencala, 1993).
Evidence suggests that hyporheic exchange is significant in large streams of the Willamette Basin. |
Hyporheic exchange can affect both the quantity and chemistry of water in streams and adjacent aquifers. For example, hyporheic exchange can cause a significant underestimate of stream discharge if a considerable volume of water is flowing within the channel deposits at the point of a streamflow measurement. In addition, because hyporheic exchange involves not only water, but also associated chemicals dissolved in the water, constituents, such as nutrients and pesticides, may undergo important biogeochemical transformations within the hyporheic zone.
Two lines of evidence suggest that hyporheic exchange is significant in large streams of the Willamette Basin (Laenen and Bencala, 1997). First, injections of rhodamine WT dye in nine streams yielded graphs of dye concentration versus time with long recession times characteristic of active hyporheic exchange. Second, in the Willamette and Santiam Rivers, detailed measurements of water discharge revealed areas where streamflow losses and gains could not be accounted for by tributary inflows or diversions. These data are consistent with stream water entering and leaving the channel through coarse-grained riverbeds.
An assessment of 1992-95 dye injection studies in the main stem Willamette River and nine major tributaries located throughout the basin showed that the lower Santiam River had the highest potential spatial extent (area) of hyporheic storage relative to river cross-sectional area. In the lower Santiam River, which has a pool-and-riffle, gravel-bed channel, potential hyporheic storage averaged about three times that for Willamette Basin streams with silt-bed channels; the latter are not expected to exhibit much hyporheic exchange. Also, simulation of the dye injection experiment in the Santiam River by assuming no hyporheic exchange resulted in a less precise fit of the curve to the data and is further evidence that hyporheic processes are important.
Dye mixes rapidly after injection in the Santiam River in June 1995. Total elapsed time for the three photographs (left to right) was less than 5 minutes.
Detailed streamflow measurements made during periods of one to several days along reaches of the Santiam and Willamette Rivers in 1992-95 showed alternating streamflow gains and losses over short distances. In the main stem Willamette River during June 1993, gains and losses were as much as 15 percent of the total discharge and point to the likelihood of significant hyporheic exchange. The relatively large flow changes occurred between river miles 180 and 140, where the channel is gravelly and braided.
Simulations of dye injection experiments were made using the OTIS model for transport of dissolved constituents (Runkle and Broshears, 1991). An Acoustic Doppler Current Profiler (Simpson and Oltmann, 1993) was used to make multiple discharge measurements along stream reaches during short time periods.
