Deep in the Grand Canyon, slivers of sand nestle against rocks of the ages, but the future of those beaches is endangered and may be measured in a few human lifetimes or less.

Seemingly timeless, the sandy terraces afford campsites each year for 15,000 boaters and hundreds of hikers.Once, sand that swept down the Colorado River regularly replenished the slender stands of land in a ritual nature had performed since about the time the first humanoids emerged on Earth.

When the gates at Glen Canyon Dam groaned shut in 1963, those cycles of renewal ended. The sand was trapped and it settled to the bottom of Lake Powell behind the dam.

And downstream, the slower-flowing river gradually but inexorably nibbled away at the remaining sand. Geologist Stanley Beus became convinced that the beaches would be doomed within decades.

His judgment proved premature, however, Beus acknowledged recently. A Northern Arizona University faculty member for 26 years, Beus began measuring the canyon beaches in 1982.

Twenty years after Glen Canyon's completion, record precipitation filled Lake Powell and had to be released. The same scenario occurred the following year, 1984.

The floodwaters rampaged through the canyon in volumes far greater than those typical of the pre-dam years.

"It rebuilt the beaches," the scientist said. "The flooding tore away some beaches but it built up more than it took down. I was surprised because I thought it would wash them all away."

Thereafter, though, the erosion resumed. Some of the beaches have subsided as much as four feet, others so much that the loss no longer can be measured.

"They're under water," Beus said.

If flash floods equal to those of the mid-1980s were to recur frequently, he said, "the beaches could be preserved at some level forever.

"But it may be 50 years before we get another one, and many of the beaches may not last that long. They would be considerably depleted in 50 years."