At least 20,000 chinook salmon and other fish have died in Northern California’s Klamath River in the last two weeks, but federal officials are unwilling to attribute the deaths to the Bush administration’s decision to divert water away from the river this year and into an irrigation project in southern Oregon. U.S. Fish and Wildlife Service Director Steve Williams said fish health began to improve on Saturday, well before emergency releases of water from Oregon’s Upper Klamath Lake reached the California “dead zone.” He and other federal officials say that fact suggests that low water flows weren’t responsible for the fish kill, which biologists call the largest in memory. But environmentalists, Native American tribes, and fishing groups discount that notion, saying the death rate began to decline simply because the majority of fish were already dead by Saturday. To call attention to their cause, the salmon advocates joined together yesterday with Rep. Mike Thompson (D-Calif.) to deliver 500 pounds of dead chinook to the Interior Department’s offices in Washington, D.C.