PORTLAND, Ore. (AP) -- A new state study has found that nearly 70 percent of the rivers and streams in the Willamette Basin are too warm to protect salmon, trout and other cold-water fish.

The Department of Environmental Quality assessment also found that almost half the 11,000-plus miles of streams in the basin have a shortage of tree cover and streamside plants, hurting aquatic life by boosting water temperatures, erosion and water pollution.

The study sampled 650 sites throughout the Willamette Valley, consolidating work done by government biologists, watershed councils and Oregon State University researchers.

On the plus side, streams on forest lands, still the main land use in the basin, are in relatively good shape. So are rivers and streams in relatively unpopulated areas, including basins for the Clackamas and McKenzie rivers and the Willamette's Coast Fork.

But overall, DEQ Director Dick Pedersen said the assessment shows that work remains to restore the Willamette and its tributaries. The basin includes most of Oregon's farms, cities and residents.

"People encroach on streams -- they have for a long time," Pedersen said. "And the streams are feeling that pressure."

City streams account for about a tenth of the stream miles in the valley -- and a fifth of the most biologically damaged streams, the study's authors estimate. Agriculture covers roughly a third of the valley's streams -- and nearly two-thirds of the most damaged streams.

Researchers also found strong connections linking damaged streamside vegetation, higher temperatures and impaired stream life.

A thick tree canopy shades water, with fallen trees adding habitat for insects and fish. Thick vegetation also helps stop and filter polluted runoff, said Aaron Borisenko, the laboratory's watershed assessment manager.

Focusing on that vegetation "seems to be a fairly simple way of getting at some big problems," Borisenko said.

Cities are already under pressure to restore streams. The Willamette system is listed as impaired under federal law because of high temperatures. That's prompting sewer agencies to spend more ratepayer money planting trees and native shrubs along streambanks.

Farmers are subject to a state law that encourages buffer zones and bars them from degrading streams or removing streamside vegetation. Money from the federal government and the Oregon Watershed Enhancement Board helps with restoration projects, said Ray Jaindl, administrator of the Oregon Department of Agriculture's natural resource division.

Still, the question of whether enough is being done is fiercely debated. The Oregonian says it has taken lawsuits to force DEQ to crack down on polluted industrial stormwater.

Farmers have a lot of leeway in how they protect streams, and enforcement of the law is lean, said Travis Williams, executive director of Willamette Riverkeeper.

The Farm Bureau cautions against exaggerating the negative effects of farming. A separate DEQ study that looked at toxics in Willamette Basin rivers found widespread presence of herbicides, but none violated water quality criteria.

It's not clear whether expanding streamside vegetation -- adding costs and taking agricultural land out of production -- will knock down temperatures enough to make a difference, said Katie Fast, the Farm Bureau's director of governmental affairs.

The issues are playing out daily in the Tualatin River basin, where 800 miles of streams flow from the Coast Range through cities such as Hillsboro, Tigard and West Linn to the Willamette.

DEQ's assessment ranked the Tualatin first among 12 subbasins for disturbed streambeds, excess sediment and human disturbance of streamside areas, citing "extensive" water quality impairment.

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