GRANTS PASS, Ore. — A federal report on what caused the deaths of 33,000 salmon in the Klamath River in September 2002 points to a large return of fish and low water levels as two primary factors.

The report released Tuesday by the U.S. Fish and Wildlife Service office in Sacramento, Calif., said the low river flows — particularly out of Iron Gate Dam — failed to draw adult fish upstream.

That left large numbers of fish crowding into warm pools, where they succumbed to the fast spread of deadly gill rot diseases, according to the report. The report also said the death toll was the largest on record for adult salmon in the Klamath River and possibly the Pacific Coast.

"Flow is one of the major factors they identify, and it is the one we can do something about," said Kristen Boyles, a lawyer for Earthjustice. Boyles represents commercial salmon fishers and environmentalists in legal battles over the Klamath.

The report is likely to become evidence in the trial of the Yurok and Hoopa tribes' lawsuit against the federal government claiming its decision to restore irrigation to farmers violated responsibilities to sustain salmon harvests.

In 2001, Klamath Basin farmers pried open irrigation gates and formed a bucket brigade to dump water into irrigation ditches after the government cut off water to benefit salmon and other fish.

Interior Secretary Gale Norton's subsequent decision to divert water from the Klamath River to 1,400 farms was criticized by environmentalists and tribal leaders, who said it was the reason for the fish kill.

Environmentalists and commercial fishers said they hoped the report would push the Bureau of Reclamation, which controls the split of water between the river and farmers, to allocate more water to the river.

Reclamation spokesman Jeff McCracken said that was unlikely, adding that the Klamath Project accounts for only one-third of the flows where the fish died.