Dam Cuts Off Flow of Sand : Beaches in Grand Canyon Periled

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

The seemingly timeless sandy terraces afford campsites each year for 15,000 boaters and hundreds of hikers.

Natural Ritual

Once, sand sweeping down the Colorado River regularly replenished the slender stands of land in a ritual that nature had performed since about the time that the first humanoids emerged on Earth.

But, 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.

Downstream, the slower-flowing river gradually but inexorably nibbled away at the sand beaches, convincing geologist Stanley Beus that the beaches would be doomed within decades.

Estimate in Error

However, that estimate was in error, Beus--a Northern Arizona University faculty member for 26 years who has been measuring the canyon beaches since 1982--acknowledged recently.

Twenty years after Glen Canyon’s completion, record precipitation filled Lake Powell and had to be released. The same thing occurred the next 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.”

Beaches Subsiding

Thereafter, though, the erosion resumed. Some of the beaches have subsided as much as 4 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 occur frequently, he said, “the beaches could be preserved at some level forever.

Depleted in 50 Years

“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.”

At least in theory, the federal Bureau of Reclamation, which controls the dam, could spill water into the gorge whenever it wished. Its job, though, is to store as much water in Lake Powell as possible.

“Maybe someday we’ll find out enough to say, ‘Here’s a possible way of maintaining the beaches.’ We haven’t yet,” Beus said.

On an average day until 1963, about 380,000 tons of suspended sediments, mostly grains of quartz, tumbled down the river, and the Colorado’s flow ranged seasonally from 5,000 to 58,000 cubic feet a second.

During the flooding of 1983 and 1984, flows reached 96,000 cubic feet a second.

Sediments Choked Off

After those years, the stream of sediments again was choked off to nearly nothing, and the river flow rarely reached 28,000 cubic feet a second.

According to other studies, essentially none of the sand that “reworked” the beaches during the flooding slipped past the dam. Rather, it was whipped up from the river bottom.

Old Sand

“When you have a high velocity, that causes turbulence, and turbulence picks up sand,” Beus explained. “But where did that sand come from?

“Some of it is old sand that probably has been there a long time, and some of it no doubt is supplied by the tributaries, small though they are, that come in below the dam.”