found that water emerging from gravel bars on the Clackamas River
in Oregon was cooler
than water in the main channel during the hottest part of the day,
but that adding gravel bars was unlikely to cool the whole
PORTLAND, Ore. June 8, 2011. 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.
in the June 2011 issue of Science Findings, a monthly publication
of the station—is 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
Gordon Grant and Oregon State University colleagues Barbara Burkholder
and Roy Haggerty examined the effect of subsurface water flow through
riverbed sediment—a 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 efforts—aimed
primarily at reversing changes in river channel morphology that
have resulted from sediment transport being interrupted by the dams—allowed
the researchers to explore whether doing so had any measurable effect on
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.
team hypothesized that the continual cycling of subsurface water
through the riverbed—during which cool nighttime water would
travel through the gravel bar, exiting and mixing with the stream
during the warmer daytime—would have a “buffering” effect
that would keep the river’s daily peak temperatures down, but not
necessarily change the river’s overall mean temperature. To explore
their hypothesis, they mapped the locations of gravel bars along
a 15-mile stretch of the
river and documented the temperature of water cycling into and
out of each of them.
They found 52 temperature differences within
the stretch of the Clackamas, with temperatures at these locations
from 1 to 4 degrees
cooler than the main channel. The researchers were then able to
link the cooler
areas with specific gravel bar features and with specific times
and locations within the Clackamas to create models that depicted
the subsurface flow
patterns—ultimately revealing that a very small percentage of the
river’s water actually passed through the gravel bars, making any
overall effect on the mean temperature minute.
Results showed a hundredth of a degree of temperature change through
a single bar,” said Grant. “Not much.”
This finding suggests
that gravel augmentation alone is not likely to have a significant
temperature-mediating effect in large rivers.
However, the work demonstrated that gravel augmentation may provide
benefits to fish and small invertebrates by creating cool areas
within rivers where they can seek refuge during hot weather.
read the June 2011 issue of Science Findings online, visit http://www.treesearch.fs.fed.us/pubs/37952. ”
The PNW Research Station is headquartered in Portland, Oregon.
It has 11 laboratories and centers located in Alaska, Oregon,
and Washington and about 425 employees.