HR: 12:05h
AN: H12B-08    [Abstracts]
TI: Effects of River Discharge on Hyporheic Exchange Flows in Large Gravel-Bed Rivers: An Empirical Study
AU: * Hanrahan, T P
EM: tim.hanrahan@pnl.gov
AF: Pacific Northwest National Laboratory, P.O. Box 999 MS K6-85, Richland, WA 99352 United States
AB: Current in situ understanding of exchange flows in riverbeds, and the ecological implications, has been largely derived from research in small streams. Comparatively little research has been conducted in large rivers, making it difficult to "scale-up" (spatially and temporally) our knowledge from small streams to understand observations and develop predictive models for large rivers. The studies conducted on small streams typically encompass too few sites, over too small of a longitudinal scale, with discrete sampling events (versus continuous sampling), resulting in an understanding of processes that is not directly transferrable to the study of much larger rivers. In addition, very few of these studies evaluate the effects of variable river discharge on riverbed exchange flows. We studied exchange flows between the river and riverbed at 14 sites distributed throughout 160km of the Snake River in Hells Canyon, Idaho, USA. Interactions between river water and pore water within the riverbed were quantified through the use of self-contained temperature and water level data loggers suspended inside of piezometers. The data were recorded at 20 min intervals over a period of 200 days when the mean daily discharge was 218-605 m$^{3}$ s$^{-1}$, with hourly stage changes as large as 1.5 m. The effects of discharge on vertical exchange between the river and riverbed were evaluated through measured hydraulic and temperature gradients, and the application of a numerical model. The vertical hydraulic gradients (VHG) between the river and the riverbed suggested the potential for predominantly small magnitude vertical exchange. The VHG also showed little relationship to changes in river discharge at most sites. Despite the relatively small VHG at most sites, the results from the numerical modeling suggested that there was significant vertical hydrologic exchange during all time periods. Our observations confirm the presence, and quantify the relative importance, of both advective and diffusive riverbed transport processes. The combined results of temperature monitoring and numerical modeling indicated that only two sites were significantly affected by short-term (hourly to daily) large magnitude changes in discharge. Although the two sites exhibited acute flux reversals between river water and hyporheic water resulting from short-term large magnitude changes in discharge, these flux reversals had minimal effect on mean daily riverbed temperatures.
DE: 1894 Instruments and techniques
DE: 1829 Groundwater hydrology
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting