`Gravel Popsicles' aid researchers in river studies.

Byline: MIKE STAHLBERG The Register-Guard

BLUE RIVER - "Gravel Popsicles" may reveal whether last spring's high turbidity in the South Fork and main stem McKenzie rivers damaged salmon spawning beds.

Oregon State University researchers working under the U.S. Forest Service's Pacific Northwest Research station are using "freeze core" technology to draw samples from gravel beds in the two rivers.

The work involves driving three closely spaced metal tubes into the gravel beds, then pumping liquid carbon dioxide under pressure into the tubes, freezing the water and gravel around and between the tubes.

"They then pull out what amounts to a gravel Popsicle, which can be sliced into a series of sections and analyzed both for course gravel and very fine silt and clay," said Gordon Grant, a U.S. Forest Service hydrologist who is overseeing the graduate student researchers.

Researchers will compare "core samples" taken from sections of the two rivers exposed to the turbidity with samples taken from areas that were not exposed to it. In the South Fork, gravel beds above and below Cougar Dam are being analyzed. In the main stem, researchers are gathering samples from above the confluence with the South Fork to as far as five miles below the confluence.

The turbidity was created by mud and silt flushed out of the reservoir behind Cougar Dam on the South Fork McKenzie when the reservoir was drawn down to allow access for construction crews installing temperature control and fish passage facilities at the dam.

The draw-down left the McKenzie River badly discolored, disrupting the first month of the trout fishing season.

The river water eventually cleared, but biologists want to know how much mud and silt remained behind on the river bottom.

"We're trying to get a sense of whether the releases from Cougar Dam this last spring resulted in more permanent changes to the river channel," Grant said. "We're looking to see if any of that fine sediment is actually stored in the gravel itself or whether it `blew through' the system.

"The way that fine sediment in gravel affects fish is not well understood," he said. "It could potentially change the property of the gravel, the way that water might move through gravel and aerate redds." (Redds are the gravel "nests" in which spawning fish deposit their eggs.)

Grant said it's too soon to draw even preliminary conclusions, as researchers have been collecting core samples for only a couple of weeks.

The U.S. Army Corps of Engineers, which operates Cougar Dam and which is installing the temperature control towers there to improve fish habitat, is spending "about $20,000" for the research, according to Chuck Willis, a fishery biologist for the Corps.

The results of the study - along with information other researchers are gathering about the impact the turbidity may have had on insect life in the river - could help the Corps decide how to manage future draw-downs of the reservoir, Willis said.

If the researchers do detect a problem in the gravel beds exposed to turbidity, Willis said, they will be sent out to gather more "gravel Popsicles" next spring in an attempt to determine whether high winter flows "have displaced all of that sediment and basically replenished the gravel."

CAPTION(S):

Oregon State University A "gravel Popsicle," or frozen core sample, collected from the McKenzie River.