Bruce Rischar is scratching a map in the sand with a stick, using circles to represent boulders in the rapids behind him, when rubber rafts and kayaks round the bend.

Rischar, director of the University of Arizona's outdoor adventures program, is explaining the importance of planning a route through Reforma Rapid to avoid the boulders. As he glances over his shoulder at the passing boats, one of the rafts vaults over a huge rock and nose-dives into the whirlpool-like "hole" behind it.The bow of the raft stalls in the hole, and two of the helmeted paddlers are thrown into the frigid, churning whitewater.

The river sweeps one swimmer downstream onto another large rock, where he narrowly escapes being squashed like a bug by the bow of the raft that ejected him. Oblivious and drenched, he hoots with delight as friends drag him back into the boat.

"He had no idea how close he was to serious injury," said Rischar, who led a recent outdoor adventures trip to the Salt. The organization offers instructional trips to UA students, faculty, staff and their guests.

"He was really lucky," Rischar said. "But 99 times out of 100, people get away with stuff like that down here."

Welcome to Salt River Canyon, which in recent years has become a mecca to U.S. rafters and kayakers seeking early season whitewater on an untamed river. On spring weekends, the upper Salt is clogged with clusters of ungainly rubber rafts and sleek kayaks waiting to shoot the rapids.

On peak weekends, more than 500 people a day brave the rapids just below the Arizona 77 bridge north of Globe. In a good year - and this year is considered the best year since 1993, due to the abundant snowpack in the White Mountains - the fun can go into June.

The river rewards its guests with a thrilling ride through a wild desert canyon. But there is growing concern that the throngs are, as U.S. Forest Service river ranger Kevin McCombe put it, "loving the wilderness to death."

Campers gathering firewood strip the beaches of vegetation, leaving fire rings and burned wood behind.

Sacred Apache sites have been vandalized. And overcrowding diminishes the quality of the outdoors experience for all.

"The Salt River used to be this undiscovered little jewel in the Southwest," said McCombe, one of three Forest Service rangers who patrol the river.

"But the Salt has been discovered, and it's gotten more crowded," he said. "This is the first river that has runoff in the Rocky Mountain states, and now this is where everybody goes to do their winter boating."

Most Salt rafters spend part of a day running portions of the 10-mile stretch just below the Arizona 77 bridge. Most of them ride with one of the four commercial outfitters who ply the waters of the upper Salt.

More adventurous river-runners take multiday trips into the canyon's wilderness section, which starts 20 miles below the bridge and continues for 32 miles. The 52-mile ride from the Arizona 77 bridge to the Arizona 288 bridge can take up to five days.

"In the last 10 years, and mainly in the last five years, we've seen a dramatic increase in usage," said Jon Cooley, director of the wildlife and outdoor recreation division at the White Mountain Apache tribe.

"It's getting to the point where we're reaching our carrying capacity, from a recreational standpoint," he said.