By Eric M. Johnson

SEATTLE (Reuters) - Washington state must repair road culverts that are blocking salmon from swimming to spawning areas because the pipes violate fishing rights protected by tribal treaties, a federal appeals court ruled on Monday.

The ruling marks a major victory for 21 tribes joined by the U.S. government that sued Washington state in 2001, arguing that hundreds of culverts block salmon from more than 1,000 miles (1,600 km) of streams in western Washington.

"The Indians did not understand the Treaties to promise that they would have access to their usual and accustomed fishing places, but with a qualification that would allow the government to diminish or destroy the fish runs," Judge William Fletcher, of the 9th U.S. Circuit Court of Appeals, wrote in a 59-page opinion on Monday.

Three judges unanimously affirmed a lower court's 2013 order that Washington state correct its road culverts because they violated the Stevens Treaties of 1854-55, the opinion says. Under the treaties, the tribes relinquished huge tracts of land in exchange for a guaranteed right to off-reservation fishing.

The lower court held that the culverts have caused the size of salmon runs in Washington state to diminish and therefore violated the state's obligations under the treaties, according to Monday's ruling.

Culverts are essentially large pipes that allow streams and excess rainwater to flow underneath roads. They can block mature salmon from reaching spawning grounds and offspring from returning to the sea if they become damaged or clogged with debris.

The Lummi Nation, among the plaintiffs in the case, said after Monday's ruling that repairing the hundreds of offending culverts would cost roughly $2.4 billion.

"Our families, our culture and our economy rely on the ability to harvest fish," Tim Ballew II, chairman of the Lummi Indian Business Council, said in a statement.

Fletcher, the appeals court judge, also sided with the lower court's order that the state correct most of its high-priority barrier culverts within 17 years, and fix the remainder during unrelated construction projects or after they give way to age.

A spokesman for Washington state's Attorney General said: "We are reviewing the decision with our clients, the state agencies affected by the ruling, to determine next steps."

The ruling marked the second major decision over salmon swimming the waters of Washington state. In May, a federal judge said the U.S. government's latest plan for offsetting the harm to salmon from dams in the Columbia River watershed violates the Endangered Species Act.

(Reporting by Eric M. Johnson in SEATTLE; Editing by Paul Tait)