As global warming bears down on our Western rivers and watersheds, it threatens one of the great symbols of Western abundance: wild salmon. With each passing year, their numbers have dropped precipitously. This decline is believed to be in part the result of warming temperatures in streams and rivers.

Earlier this month, government fishery managers moved toward a ban on salmon fishing off the California and Oregon coasts because of the diminishing numbers of chinook salmon.

If we hope to save the salmon, we must do two things: Stop the rise in greenhouse gases as quickly as we can and secure our waters' health against the warming that has begun and will continue. This is a river-by-river job, and each river matters. But there is one part of the job that is critical — the piece that unites sportsmen, biologists and everyone else who cares about salmon.

The biggest, wildest, highest, coldest, healthiest and best-protected salmon habitat left south of Canada spans millions of acres and thousands of stream miles in central Idaho, eastern Oregon and southeast Washington in the headwaters of the Snake River. It is Noah's Ark for salmon — the haven they need to reach to survive and carry on.

Scientists believe the salmon that spawn in this place probably have the best chance of any salmon populations in the Lower 48 states to adapt to, and thus survive, global warming. This habitat, nearly all above 4,000 feet in elevation, will stay cool even as temperatures rise in other areas. It will give salmon the firmest footing from which to self-adapt in the face of warming. And because the area is protected as wilderness and public land, it probably will face less development pressure and could offer refuge for years to come.

In the face of the great flood, Noah had to build an ark, but this one comes already made. All we need to do is help the salmon get there.

The heart of the refuge lies in the Salmon River Mountains high above the Pacific Ocean, hundreds of miles from the coast. But the route between the ocean and the spawning ground — the ark — is choked by eight dams, which kill as much as 90 percent of the area's native salmon as they journey out to sea and back again.

If salmon are to survive climate change, four of these dams on the lower Snake River must go. Once the dams are removed, the salmon would be able to reach the ark, and scientists give such a plan a 50 percent to 90 percent probability of restoring productive populations. If the dams stay, the salmon will lose their best chance to survive global warming.

It is less expensive to remove these four dams than to keep them. The modest electricity benefits they offer to local wheat farmers can and should be replaced by clean energy sources, such as wind and solar power.

This does not mean we give up on salmon in southerly or lower-elevation rivers. We should continue to do everything we can to protect their habitats from logging and development.

But realistically, low-elevation rivers will warm more, putting salmon there more at risk. Filling the high-elevation ark with salmon is our best insurance policy against what global warming could do to these valuable fish.

We have reached a tipping point. Only four sockeye salmon returned to the ark last year, and in a few years the area's chinook salmon could also reach the brink of extinction. We must act now, and if we do, the odds of success are excellent.

Get out a map of America. Find the wild stretch of Idaho, eastern Oregon and southeast Washington through which the Snake River winds, a region with very few roads or towns, nearly all of it public land. This is Noah's Ark for salmon, the place fish must reach if they are to survive climate change. But the salmon can't do it on their own. Like Noah, we must help them to safety.

Carl Pope is executive director of the Sierra Club. He wrote this commentary for the Los Angeles Times.