DEXTER – A missing boater is presumed dead after a canoe capsized Saturday afternoon on a flooded Black River, according to Christopher A. Gracey, the town’s assistant fire chief.

An accompanying paddler was rescued shortly after the 4:30 p.m. call. Town firefighters were already near the scene at the Dexter boat launch, and the Jefferson County Strategic Tactics and Rescue team met them within minutes.

Shortly before 10:30 p.m., crews ended their rescue efforts and said they would begin searching for the boater’s remains at 4:30 this morning. Rescuers did not release the names of the victim or the victim’s companion in the canoe.

Emergency workers expressed frustration over a death they said could have been prevented. On the last day of the wettest April the region has seen in more than six decades, theBlack River’s danger was well publicized.

The National Weather Service issued a flood warning for the Black River on Thursday, warning of rapid waters and minor to moderate flooding. That warning remains in effect until Monday.

Mr. Gracey said the paddlers shouldn’t have been navigating the river this weekend.

“”The Black River has always been a very extreme river, even on its normal day,”” he said. “”This just makes it worse.””

Water levels on the flooded boat launch spiked after boards atop village dams collapsed. A nearby alarm meant to signal a dam failure didn’t go off because the collapse was controlled. The dam was designed so that boards collapse top to bottom.

Brownville Fire Chief Steven L. Mott was one of the rescuers assembled on the already-flooded boat launch next to a recently broken dock that was partly submerged in water.

“”Probably within four minutes, it came up about a foot,”” he said near a fire hydrant that was nearly submerged. “”People were standing and taking pictures, and they turned around, their feet were wet.””

Water levels quickly subsided, but not before most drivers parked on the launch’s nearly full parking lot fled.

The Jefferson County Sheriff’s Department, the U.S. Coast Guard, and the state Department of Environmental Conservation assisted in the search.

The drama in Dexter came on a day when large swaths of Jefferson, St. Lawrence and Lewis counties remained flooded. On Route 3 in Pamelia, for example, water was only a couple of feet from reaching the road.

Paddlers should avoid the river in the next few days, Mr. Gracey said.

“”It’s an ever-changing river,”” he said. “”It’s a stronger river than most.””