SACRAMENTO — The American River once hosted thousands of steelhead migrating upstream from the ocean in three separate runs. Today it's down to just two runs of a few hundred fish.

The Sacramento was the only river in western North America with four salmon runs. They numbered in the millions — so numerous that American Indians and settlers could catch a salmon dinner with their bare hands. Now one run is gone, and two are endangered. The fourth could join them soon.

Restoring a fragment of that spectacle to the Central Valley is the goal of rules proposed by the National Marine Fisheries Service. The service wants, among other things, restoration of winter- and spring-run salmon above Shasta Dam on the Sacramento River, and steelhead above Folsom Dam on the American River.

Combined, the fish transit order is considered the biggest of its kind in U.S. history.

Making it happen presents huge financial and engineering challenges. Costs could exceed $1 billion at a minimum – more than 10 times the original construction cost of both dams.

"It's pretty substantial, the amount of work that's required," said Mike Chotkowski, regional environmental officer at the U.S. Bureau of Reclamation, which operates the dams. "We still haven't even determined whether it's feasible."

The fisheries service says that without restoring access upstream, it's likely the three fish species will go extinct. Climate change means it will be harder to maintain cold-water habitat below the dams, so they must have access to better habitat.

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