NORTH FORK, Idaho — The Middle Fork of the Salmon River, one of the wildest rivers in the contiguous United States, is prime fish habitat. Cold, clear waters from melting snow tumble out of the Salmon River Mountains and into the boulder-strewn river, which is federally protected.

The last of the spawning spring-summer Chinook salmon arrived here in June after a herculean 800-mile upstream swim. Now the big fish — which can weigh up to 30 pounds — are finishing their courtship rituals. Next year there will be a new generation of Chinook.

In spite of this pristine 112-mile-long mountain refuge, the fish that have returned here to reproduce and then die for countless generations are in deep trouble.

Some 45,000 to 50,000 spring-summer Chinook spawned here in the 1950s. These days, the average is about 1,500 fish, and declining. And not just here: Native fish are in free-fall throughout the Columbia River basin, a situation so dire that many groups are urging the removal of four large dams to keep the fish from being lost.