Global warming is expected to further weaken wild chinook salmon populations by changing the temperatures and flows of major river systems, according to a study published Thursday by the National Academy of Sciences.

Warmer waters in the summer and early fall are expected to cause more disease, stress and die-offs, while rain-swollen rivers in warmer winter months could flush out salmon eggs from spawning gravel.

The study, by federal and University of Washington scientists, offers a sobering perspective on the challenges that climate change creates for the multibillion-dollar regional effort to restore wild salmon runs.

Many of the restoration planners use models that do not account for warmer temperatures that are expected to shrink the size of the annual snowpack and alter run-off patterns.

Thus, they may generate "misleading predictions of the relative benefits of different recovery strategies," the study states.

The study focused on the effects of global warming on the chinook populations of Washington's Snohomish River basin. The researchers concluded that by 2050 wild chinook populations would decline by 20 percent to 40 percent in the Snohomish. The range of decline depends on which of two computer models was used in the analysis.

Researchers expect the chinook declines would be similar in other Western Washington and Oregon drainages, though they noted that some salmon could perhaps learn to alter their migration timing to improve survival rates.

The study did not attempt to assess the effects of climate change on other salmon species, or salmon that spawn in the Columbia River basin. But global warming also is expected to make life more difficult for those fish.

"These hydrological changes are not going to be good for any [salmon] species," said Mary Ruckelshaus, a National Oceanic and Atmospheric Administration (NOAA) fisheries scientist who was one of seven co-authors of the study.

Global warming is expected to cause the biggest problems for salmon that spawn in the more remote, higher-elevation river basins, according to the study. Salmon eggs in those areas would be among the most vulnerable to sudden surges in winter flows triggered by high-elevation rains. As spawning survival rates there decline, salmon may concentrate more in the mid- to lower reaches of the rivers, according to the study.