Thousands of fish may have died in the Upper Youghiogheny River in what one official called "the perfect storm" for a fish kill.

After water was released through the Army Corps of Engineers' Youghiogheny Dam last week, Bill Holliday, who lives a half mile down the river from the dam in Confluece, Somerset County, alerted engineers at the dam and the Pennsylvania Fish and Boat Commission of the kill.

"There were thousands ... of the little alewives, three to six inches long, lining the banks downriver from the dam," he said. "There were also hundreds, maybe thousands, of walleyes, perch and bluegills lying belly up and wriggling as they died."

Mr. Holliday said he saw many walleyes, 13 inches to 20 inches long, in the weeds along the river, and at least one that was more than 30 inches long following the large release of water.

"This was a huge release, and for this time of year it was unusual," said Tom Qualters, assistant regional supervisor for the Fish and Boat Commission.

The fish kill began Dec. 12 as the corps drew down the Youghiogheny Reservoir at the request of the Pennsylvania Department of Transportation to facilitate removal of debris from the former Route 40 bridge pier. The bridge was replaced in 2005, but rebar jutting above the lowered lake surface posed a hazard to boaters.

An unusual series of events precipitated what Jeff Hawk, public affairs officer for the Corps of Engineers, called "the perfect storm" for a fish kill.

"The water was already lower than usual for this time of year," he said. "The low water is colder, pushing fish toward the deepest point, which is warmer, right at the dam. The gates were opened to release water so the contractors could do their work, causing a current through the dam.

"The unusual cluster of fish was there, the alewives naturally followed the current, and the game fish followed them."

As the fish moved from deep water through the dam's outlets, the rapid change in pressure caused their air sacs to burst, eventually killing them.

The outflow was stopped shortly after Mr. Holliday notified the Corps of Engineers of the kill.

Workers from the Corps of Engineers and D/R Hydro Co., which generates electricity at the dam, were still counting dead fish yesterday. The magnitude of the fish kill may be difficult to assess, however.

Mr. Holliday said most of the dead fish didn't begin to surface until they had washed below the Great Allegheny Passage bike trail bridge in Confluence, about a quarter mile downriver from the dam. Mr. Hawk and Mr. Qualters said fish killed in this manner tend to perish slowly and may wash miles downstream before dying.

Long-term damage to the local fishery is expected to be minimal, said Mr. Qualters, but he was uncertain who would be held responsible for the fish kill.

"One of Youghiogheny River Lake's purposes is environmental flow augmentation to the Youghiogheny River and its tributaries, so we take any impacts to the environment and the wildlife seriously. We'll continue to look into this," said Mr. Hawk.