Rescue teams spent a second day Saturday looking for an unidentified kayaker presumed lost on the Potomac River, plying the rapids swollen from heavy rainstorms with swift boats, searching an eight-mile stretch below Great Falls by helicopter, and using thermal imaging along the banks and footpaths for any sign of a man seen on video struggling in the rough water Friday afternoon.

Meanwhile, two kayakers were rescued from the Patuxent River in Prince George’s County on Saturday evening after one fell out of her kayak. Rescuers used cellphone data to help find them, said Marc S. Bashoor, the county fire chief.

In Montgomery County, police spent the day interviewing hikers and kayakers and looking for cars that might have been left in parking lots along the Potomac shore overnight. Police released photographs of a red kayak that was recovered downstream shortly after a witness on the Observation Deck at Great Falls shot the video and called 911. They also released photos of a white aluminum paddle and a Spiderman-themed gear bag that were recovered. The equipment, seasoned kayakers say, is outdated, inappropriate for such treacherous high-water levels and lacking standard safety features.

After several fruitless hours of searching and fighting rising water, turbulent currents and floating debris, the rescue teams called off the search Saturday afternoon. They will resume Sunday, said Pete Piringer, spokesman for the Montgomery County Fire and Rescue Service.

“We did an extensive search, and we didn’t turn up anything,” Piringer said. “Typically, the people that boat in this area are expert boaters, Olympic-level people, so it’s not unusual to see people out here every day of the year. But these river conditions are dangerous, even for the expert boaters.”

National Weather Service data indicated that the river had been above the danger level for boaters since Wednesday.

And that, said a veteran kayaker, can be a recipe for disaster for inexperienced paddlers.

At between five and six feet, Friday’s level, the Potomac runs as fast and furious as the Colorado River in the Grand Canyon.

And around Great Falls, the steepest fall line of any river on the East Coast, even the most seasoned kayakers with the latest safety equipment think twice before daring to ply the frothing waters and the standing wave that appears when the water runs this high on a portion of the river paddlers call the Central Chute.

Veteran kayaker Howard Morland, 71, found the empty kayak. He had gone out Friday afternoon to surf the wave on Central Chute in a short, sturdy, freestyle rodeo boat designed for experienced kayakers to do tricks and surf. He put in at Angler’s Inn and, as he was heading upstream, stopped at Sherwin Island just below the falls to fix his drain plug. That’s when he saw the overturned empty red kayak rush past.

He brought it to shore, and said it was too long and narrow for conditions.

Because no one saw the man in the video actually go into the water, Piringer said, police can not rule out the possibility that he might have swum ashore.

“But no one’s reported anyone missing,” he said. “No one’s reported that they lost their boat.”

In the Prince George’s incident, Bashoor said that a man and woman had gone about five miles upstream when the woman fell into the water.

He said the man used his cellphone to call 911. But they were at a point where the river runs through a largely rural area, with densely wooded and swampy banks, and the pair could not specify their location, Bashoor said.

“They knew only that they were on the Patuxent,” the chief said.

He said rescuers were able to get their cellular carrier to ping their telephones, which helped in triangulating their location.

With the help of a state police helicopter and an off-duty firefighter who happened to be nearby, he said, “we were able to get their exact location” within an hour of the first 911 call.

He said the woman had turned over her kayak and could not get back in it. She then became pinned against a tree in the water.

“They got to the point where they couldn’t go any further and needed help,” Bashoor said.

A rescue boat from Anne Arundel County reached them. Although rescuers were concerned about hypothermia, both kayakers declined to be taken to hospitals.

Bashoor said the matter emphasized the need for kayakers to be familiar with the waterways they use.