The Youghiogheny River stopped flowing Monday afternoon in the half mile between the Youghiogheny Dam near Confluence and where it merges with the Casselman River and Laurel Hill Creek in southwest Somerset County.

For about seven hours, the Yough was more riverbed than river because the 56-year-old Army Corps of Engineers dam was shut down to allow for repairs to a 15-year-old D/R Hydro Co. electric generating facility.

By 6 p.m. Monday, the corps was able to partially open one of the gates to allow a flow of approximately 125 cubic feet of water per second to replenish the river and reestablish shallow coverage from bank to bank. But that is only about one-quarter of the usual flow of 500 cubic feet per second, and gravel bars and rocks normally covered by water remain exposed.

That low flow level will continue through Sunday or Monday, the corps announced yesterday, to allow the hydro power producer to replace concrete that has been chipped away inside the dam tunnel.

"They've had to extend the project for a couple of days because they found out after the water was shut off that they needed a 1-inch-thick plate instead of the 5/8-inch thick plate they originally thought, and had to have one shipped from Baltimore," said Werner Loehlein, chief of the corps' water management section.

When repairs are completed, the discharge gates will once again be closed to allow for removal of work scaffolds, equipment and temporary damming structures inside and at the outflow end of the dam tunnel.

When the corps completely closed the gates Monday, the river quickly dried up, exposing algae-green rocks, gravel bars and mud flats, and turning the normally robust, 100-yard-wide river into a series of small pools connected by almost invisible trickles of water.

A handful of fishermen crowded around some of the deeper river holes where trout were artificially schooled.

Ducks and Canada geese waddled through the muddy rocks snapping up crayfish and stranded alewife, sculpins and perch. Thousands of small fish appeared to have been fatally injured by pressure changes as they were sucked through the dam intake at the bottom of the lake and discharged into the outflow pool.

Loehlein said the work has been scheduled for months but could only be done during a three-week period at the end of October and beginning of November when it would have the least impact on down river recreation.

It also would not have been allowed if the combined flows of the Casselman River and Laurel Hill Creek were not at least 450 cubic feet per second -- a rate that ensures enough water for aquatic life on the reaches of the Youghiogheny River below Confluence as well as for several public drinking water intakes.

"Since the water quality has improved and we have become more environmentally aware, we have to balance the needs to do maintenance work with the environmental impact," Loehlein said. "We've made every effort to fit in this work with those environmental concerns."

Rosemary Reilly, a corps biologist, said discharges from the dam were gradually reduced over two days to minimize the impact on the river's fish and other aquatic life.

"There was a step-down process before the total shutdown so the cut off of the flow was not so dramatic," Reilly said. "That allowed the fish to gradually concentrate in the main channel."

Although she hasn't visited the river since the dam turned off its discharge, a similar shutdown for two weeks last year caused the stranding of sculpins, and perch but few larger fish, Reilly said.

Before last year's shutdown in October and November, the dam's outflow also was closed for repairs six times for more than 18 days in 1962, for three days in 1963, three days in 1968 and for more than 20 days on three occasions in January, February, November and December 1969.