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NW Fishletter #354 February 1, 2016

[3] Floodplains Key To Cold Water Refuges, Research On Willamette Shows

Over the last decade, the Willamette River--one of the Columbia River's largest tributaries--has exceeded maximum temperature standards for water quality each year during the summer.

Now two university researchers, Stan Gregory and David Hulse, have found floodplains are key to the presence of cold-water habitats, which are used by native fish during periods of high water temperature.

Gregory, an Oregon State University emeritus professor of fisheries and stream ecology, and Hulse, a professor of landscape architecture at the University of Oregon, have worked together for more than a decade to understand river processes on the Willamette.

Between 2008 and 2015, Gregory and his OSU team sampled more than 100 sloughs, side channels and tributary mouths on the main stem Willamette to identify where cool-water habitats occur and whether fish were using them.

They discovered that 72 percent of the river's floodplain sloughs and alcoves are likely to be colder than the mainstream Willamette. About 40 percent of these areas were more than 2 degrees Celsius. (3.6 F) cooler than the mainstream.

Only 25 percent of the river's side channels, on the other hand, were colder than the mainstream, and none was more than 2 degrees Celsius cooler.

A cold water refuge must be at least 2 degrees Celsius colder than "the daily maximum temperature of the adjacent well-mixed flow of the water body," as defined under Oregon regulation.

"The majority of the cold water sloughs had conductivity values that were similar to the mainstream river, indicating hyporheic flow as the major source of cold water rather than groundwater," Gregory wrote in briefing notes to the Northwest Power and Conservation Council. He summarized his and Hulse's research at the Council's Jan. 12 meeting.

Hyporheic refers to flow through gravels at the river bottom and along riverbanks. These flows encourage biochemical processes important to water quality and aquatic habitat.

Salmonids and other native fish need not only cool water, but also adequate oxygen.

To find out whether cold-water habitats had enough dissolved oxygen concentrations, the researchers sampled these habitats and learned that 80 percent of the floodplain sloughs that were more than 2 degrees Celsius colder than the mainstream contained oxygen adequate to support salmonids and other native fish.

"One-third of the sloughs in the Willamette River meet the definition of cold water refuge and have adequate oxygen for native fish," Gregory said.

The research found that native fish, including salmonids, made up the majority of fish captured in cold-water sloughs. Gregory mentioned that salmonids were 10 times more abundant in cold-water sloughs than in warm-water sloughs.

Salmonids are considered cold-water species.

To actually help salmonids migrate through the increasingly warm waters of summer, cold-water refuges can't be too far apart.

In a 2007 study of the Willamette River, researcher Hulse reported there were "four contiguous river reaches in the study area that lack any observed or expected cold water refuge over a distance far exceeding even the most generous estimate of adult cutthroat trout and adult steelhead effective travel distance."

He estimated in the study that distances needed between refuges may be as short as one kilometer.

Hulse and Gregory say floodplain and river-restoration actions will be required to create the conditions that encourage the development of more cold-water habitats. Gregory explained that these habitats "are created by the performance of natural river dynamics."

Hulse and his research team at UO are mapping the information on the river's thermal patterns into what they call a SLICES database. The SLICES framework will provide a basis for designing river and floodplain restoration actions.

Meyer Memorial Trust, U.S. EPA and Oregon Watershed Enhancement Board funded this research on Willamette River cold-water refuges. -Laura Berg

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