Although adding gravel to a river to replace lost sediments won't likely cool the whole river channel, it can create cool water refuges that protect fish from thermal pollution, according to a U.S. Forest Service Pacific Northwest Research Station study.
The researchfeatured in the June 2011 issue of Science Findings, a monthly publication of the stationis among the first to explore the interplay between sub-surface water flow and temperature in large rivers and is helping to guide river restoration strategies in the Pacific Northwest.
In the study, which began in 2006, station research hydrologist Gordon Grant and Oregon State University colleagues Barbara Burkholder and Roy Haggerty examined the effect of subsurface water flow through riverbed sedimenta process known as "hyporheic flow"on daily minimum and maximum water temperatures. The focus of their study was Oregon's Clackamas River, which, at the time, was undergoing intensive restoration planning efforts led by Portland General Electric (PGE) as part of the relicensing process for the river's hydroelectric system. The addition of gravel to the large river as part of these effortsaimed primarily at reversing changes in river channel morphology that have resulted from sediment transport being interrupted by the damsallowed the researchers to explore whether doing so had any measurable effect on reducing "thermal pollution," or unusually high water temperatures caused by human activities like dam operation, logging, and wastewater treatment.
"Previous work suggested that water emerging from gravel bars might actually be cooler than the surrounding water," said Grant.
The research team hypothesized that the continual cycling of subsurface water through the riverbedduring which cool nighttime water would travel through the gravel bar, exiting and mixing with the stream during the warmer daytimewould have a "buffering" effect that would keep the river's daily peak temp
|Contact: Yasmeen Sands|
USDA Forest Service, Pacific Northwest Research Station