SEATTLE — What supporters are calling the largest river and salmon restoration effort in American history took a critical step forward on Thursday, when formal agreements were signed to remove four dams and revise how water is shared in the Klamath River basin in southern Oregon and Northern California.

The dams, which have provided hydroelectric power and water for farm irrigation for decades, have caused severe depletions in salmon populations in the 250-mile river, hurting Indian tribes in the area and helping force shutdowns of some West Coast commercial fishing.

In 2001, farmers were angered when irrigation was cut off to provide more water for salmon migrating up the river to spawn. The next year, irrigation was restored, only to have tens of thousands of fish die. The roller coaster increased tension but eventually led to talks. The basic outlines of the agreements became final in November 2008 under the Bush administration.

The agreements would remove the four dams by 2020 if a series of federal studies and Congressional approval and appropriations follow suit; the interior secretary is to make a final decision on removal by March 2012.

The agreements were signed by Gov. Theodore R. Kulongoski of Oregon, Gov. Arnold Schwarzenegger of California, Interior Secretary Ken Salazar, Greg Abel, the chief executive of PacifiCorp, the power company that owns the dams, and more than 30 groups representing tribes, farmers, fishermen and environmentalists. Some tribes and environmental groups refused to sign, citing questions over water flow for salmon and the timing of dam removal.

Describing the long standoff over the Klamath as “an intractable water war,” Mr. Salazar said in a conference call with reporters that the agreements “can become a template” for other water and environmental disputes. He said he was optimistic Congress would act this year.

Economics were critical to the agreements for PacifiCorp, which faced spending hundreds of millions of dollars to improve passage over the dams for salmon. The dams provide power for about 70,000 homes.

The expected cost of dam removal and basin restoration is about $1.5 billion. Oregon has already committed about $200 million. California will ask voters to contribute $250 million through a ballot initiative this fall, though supporters say the state could find other means to pay if the measure fails. Congress will be asked to supply the rest.