Two Virginia youths narrowly escaped going over a 20-foot cascade in the Great yfalls area of the Potomac yesterday morning by scrambling onto a small rock island after being thrown from their rubber raft in the rapids. Minutes later, a U.S. Park Police helicopter rescued them.

A park spokesman said there was a 99 percent chance the two would have drowned if they had gone over the cascade. Neither youth was injured in the incident.

Park officials would not release the names of the two young men -- a 17-year-old from Springfield and an 18-year-old from Burke -- who got on the river on the Maryland side just below the Army Corps of Engineers aquaduct dam around 11:30 a.m.

They had been on the river about 15 minutes when they fell out of the raft in the Spout area of the falls near the Virginia shore, a park spokesman said. They were about halfway down the Spout when they were pushed toward the rock island but 10 yeards from the 20-foot drop, he said.

The raft wound up in an eddy in the falls.

The youths were spotted by a number of park visitors, and the U.S. Park Police helicopter rescued them at 11:51, police reported.