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Presented at the NABS Annual meeting, Vancouver, British Columbia, 2004
in Geomorphology
The effect of geomorphic complexity on water temperature in a Pacific Northwest alluvial river
A.S. Arrigoni1, G.C. Poole2,5, L.A.K. Mertes1, W.W. Woessner3, S. O''Daniel1,4, S.A. Thomas2, and B.R. Boer3. 1Department of Geography and ICESS, University of California Santa Barbara, USA, 2Eco-metrics, Inc. Tucker, Ga, USA, 3Department of Geology, University of Montana, Missoula, USA, 4Confederated Tribes of the Umatilla Indian Reservation, Pendleton, Oregon, USA, 5Institute of Ecology, University of Georgia, Athens, USA
The Umatilla River, Oregon, USA once supported healthy populations of resident and anadromous salmonids (trout, salmon, and charr). Now, summertime water temperatures are stressful or lethal to salmonids, exceeding 26°C. We are documenting the influence of near-channel hyporheic exchange (subsurface flow pathways from 1 meter to 10s of meters) on the river's thermal regime. During the summer of 2003, we deployed a total of 80 thermal data loggers in recharge and discharge zones of near-channel subsurface flow pathways as well as in various types of stream channel units. Resulting data suggest that complex channel patterns and bed-forms enhance hyporheic exchange rates by creating hydraulic gradients within the near-channel aquifer and that diel variation in ground-water temperature is buffered by as much as 4°C along near-channel subsurface flow pathways. While associated upwelling water can create localized cool water pockets in channel margins, the cumulative effects of near-channel hyporheic flow pathways also appears to buffer diel temperature fluctuations in the well-mixed thalweg of the channel. Our results suggest that loss of bed-form complexity may increase the diel temperature fluctuation in streams by reducing rates of near-channel hyporheic exchange.
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