If warming trends continue unchecked, more than 20 percent of the Pacific Northwest's rivers could become too hot for salmon, steelhead and trout by 2040, according to a new Northwest Wildlife Federation report.

The report, released yesterday, predicts that rising regional temperatures could disturb the delicate balance of seasonal stream flows — making spring flows happen earlier, reducing summer flows to a trickle, or rendering winter flows so high that gravel beds used as nesting sites could get scoured away.

"Salmon in the region are struggling to survive amid dams, water diversions and development along river shorelines," said Paula Del Giudice, director of the group's Northwest Natural Resource Center in Seattle. "Global warming will add an enormous amount of pressure onto what's left of the region's prime cold-water fish habitat."

The report says climate change experts at the University of Washington consider it possible that average August temperatures could rise by 3 degrees Fahrenheit in the Northwest over the next 35 years.

That could boost temperatures in some Columbia River Basin and coastal rivers in Washington and Oregon past 69.8 degrees, generally considered the high end of the temperature threshold for cold-water fish, the report said.

Fish grow more slowly, become more susceptible to toxins, parasites and disease and often die when river waters get too hot.

Patrick Michaels, a professor of environmental sciences at the University of Virginia and a senior fellow at the libertarian Cato Institute, questioned the accuracy of any regional predictions about a complex global phenomenon.

"It is well known that there is no reliability for regional projections of global warming," Michaels said.

Patty Glick, a National Wildlife Federation climate specialist who wrote the report, said advances in computer modeling allow greater confidence in regional predictions. Her report cites research in several other regions.

Scientists at the University of Wyoming estimate a 5.4-degree increase in average July air temperatures could eliminate half of the now-viable trout stream habitat in the Rocky Mountain region.

A similar study by the Environmental Protection Agency said a 4.5-degree increase in average stream temperatures could reduce by half or eliminate cold-water fish habitat in some New England, Great Lakes and western states.

The bulk of the wildlife federation's report focuses on the Pacific Northwest, where UW researchers said average air temperatures rose 1.5 degrees during the 20th century — faster than the average global rise of 1 degree.

At the same time, yearly precipitation increased, mostly in the form of rain, while snowpack fell. Most of Washington state's glaciers are receding rapidly, and several have disappeared altogether in recent decades, the report says.

The organization's recommendations include:

• Strengthening government policies and programs to promote energy efficiency, nonhydroelectric renewable energy, cleaner-burning fuels and mass transit.

• Encouraging sound management of wetlands, grasslands, forests and agricultural lands so plant life can store excess carbon dioxide, which can trap heat in the atmosphere.

• Factoring global warming into water-resource management and planning.

• Re-engaging in international talks on curbing global warming.

Michaels suggested that population growth over the next 50 years will cause bigger problems for the environment than global warming might.

"Should you spend a lot of money and effort trying to slow or stop something that you can't slow or stop with today's technology?" Michaels asked.

But Jim Kramer, executive director of a cooperative salmon-recovery effort called Shared Strategy for Puget Sound, suggested that delaying the region's response to rising temperatures could make things worse in the future.

"It's similar to working on salmon issues," he said. "The decline of the salmon has been talked about for 100 years, and a lot of the things that were talked about 50 years ago, if instituted, would have prevented the problems we have today."