Three Montgomery County canoeists were rescued at daybreak yesterday from a tiny Potomac River island near Great Falls, after their canoe capsized and they were stranded for 15 hours in overnight temperatures that dipped to 40 degrees, U.S. Park Police reported.

David Dickson, 24, Roger Holmes, 24, and Julia Simmons, 20, were treated and released from Suburban Hospital in good condition, suffering only minor colds.

What began at 4 p.m. Tuesday as a sightseeing outing ended at 6:45 a.m. yesterday with a rescue by the park police Eagle II helicopter.

"Our biggest problem was our feet. We just couldn't feel them for six hours," said Dickson, who is unemployed and lives in Potomac. "We kept our heads down and snuggled up all night. We knew someone would find us by the time the sun came up."

Dickson, Holmes and police gave the following account of the incident:

Holmes, Simmons, and Dickson got into Holmes' green canoe in Great Falls State Park Tuesday afternoon and paddled 2 1/2 miles upstream, using binoculars to watch for waterfowl and deer.

"We've done it many times before and we know the area well," said Dickson.

As darkness approached two hours later, the canoe, after circling and heading back downstream, rounded Conn Island and swiftly headed for a low-level dam on the southern edge of the island, just upstream from Great Falls.

"Usually we canoe out there in the summer when the water isn't so high. In July and August you can go right up to the dam," Dickson said. "Last night . . .the current just took us faster than normal because the water is higher and swifter."

He said the canoe went over the eight-foot Aqueduct Dam and capsized. "It was about a three foot drop. Luckily we were able to stay together. We righted the canoe, then pushed it over to a clump of rocks and tree trunks in the middle of the river."

The canoeists then secured the boat and huddled together in it to keep warm.

"What we were stranded on wasn't exactly a land mass--just a grouping of rocks and trees," said Holmes, a professional musician and carpenter who lives in Potomac. "In fact, it must've been a beaver dam because at one point during the night a beaver came in and snooped around."

Shortly after 10 p.m. the parents of the boaters notified authorities their children were overdue and a search began. When Holmes' car was found, searchers zeroed in on the Great Falls region.

"We get involved in searches like this about a hundred times a year," said U.S. Park Policeman Philip Kramer. "People canoe around here, not knowing where they're going or what they're doing." He said a park police helicopter searched the area for several hours without spotting the canoeists.

Dickson said the trio saw the helicopter several times during the night, "but there was nothing we could do to help it find us." Finally, at sunrise, a park ranger spotted the canoeists from the shoreline.

At home yesterday, sniffing from a cold, Dickson said he learned two lessons from the ordeal. "First, don't go out there when the currents are rough. Second, tell your parents exactly where you're going. All they knew is that we were out in the Potomac somewhere."