Montgomery County dive teams will resume their search Monday for 21-year-old Bruce Yansen, who is presumed dead.

Yansen was swimming in Triadelphia Reservoir, near Brookeville, when he disappeared into the water around 5:30 Friday evening.

The Washington Suburban Sanitary Commission says the crews concentrated their search Sunday during the hours of the most favorable water temperatures and when boat traffic is reduced.

The search is now considered to be a recovery mission. But so far, there's no trace of Yansen. The search will resume on Monday.

"There's a lot of debris down there," says WSSC spokesman Jim Neustadt.

"By that I mean natural debris--rocks and limbs and tree trunks that are down there."

Divers say they have only four-to-five inches of visibility in a 4,200 acre search area. They'll resume the search Sunday morning.

Despite it's tranquil surface, experts say the reservoir, banned for swimmers, has a steep drop-off underwater, from four feet to depths of eighteen feet, without warning.

"There's like a shelf and it drops off," Neustadt says. "And when you've got a swimmer who perhaps isn't experienced or accomplished a swimmer--and suddenly they're taken by surprise walking off that little shelf."

WSSC recreation areas had been closed on Friday and Saturday during the search, but they reopened on Sunday and will be open again Monday.