A magnet for boaters, hikers and 2.5 million tourists a year, this mile-deep, multi-hued natural wonder is suffering the ravages of a new and troubling visitor: a massive water surge destroying favored sand beaches, uprooting shade trees and threatening fragile wildlife.

With four times the usual Colorado River flows unleashed by a flood-besieged upriver dam, park managers are contending with a new, dangerous twist in the rapids and an undetermined impact on the rare birds and fish at the canyon bottom.

But visitors in cars and campers poured into canyonside facilities today, and rafting guides, after a brief halt caused by the death of one weekend raft rider and a sudden increase in river flows, were allowed to resume launching 100 to 150 thrill-seekers a day down the river from Lee's Ferry.

The unusually sudden and heavy Rocky Mountain snow melt and rains upstream, which have caused havoc at several points along the Colorado River, have returned this 10-million-year-old canyon to the days 20 years ago when the Colorado often fluctuated wildly through here.

Steve Hodapp, a wildlife biologist who heads resource management for the National Park Service here, said the 1964 completion of the Glen Canyon Dam 87 miles upstream tamed the Colorado. This left dry several lovely sand beaches on which tamarisk trees grew and Bell's vireo, a small, rare bird, found nesting places.

Many trees and nests are now flooded, Hodapp said, and only time will tell how the plants and animals adjust to the shock and what the high waters do to native willows and one endangered species of native fish.

The water cascading down at 90,000 cubic feet per second from Glen Canyon Dam is so clean that it picks up sand and sediment with unusual ease, scouring beaches away and taking them downstream.

"Depending on how long we have flows of this magnitude, we may be going to see much vegetation lost," Hodapp said.

Despite the overturning of six boats, with one death and 15 injuries, on one particularly tricky point of the new rapids at Crystal Creek, guides said today that the high water had made the rafting generally safer. Park spokesman Tom Doerr said Saturday's death was the first of this season, whereas last year, with the water levels much lower, there were three deaths.

Terry McCarthy, 32, a guide for Grand Canyon Expeditions Inc., said today that it was "a delight for boatmen" to be able to explore and camp at side canyons and caves newly reached by the high waters. Hodapp expressed some concern about the impact of tourist camping in these areas, but said any harm would be short-lived if the high waters did not return next year.

Thus far, six people have been killed--five of them in Mexico--as the Colorado has risen above its usual banks from Grand Junction, Colo., to below the Mexican border. Glen Canyon Dam was forced to increase its water releases by 31 percent Tuesday, which will force downstream dams such as Hoover, Davis and Parker to raise their flows and could result in a new round of serious flooding on the California-Arizona border next week.

But here at what has been described as one of the world's seven great natural wonders, tourists in even greater numbers than this time last year hiked the canyon's rim and gawked at the surging waters far below.

McCarthy, who last ran the river two weeks ago, said the waters "were twice as high as I have ever seen them." But his passengers did not seem to mind that most of the more dangerous rapids had been softened by the excess water covering the rocks. "You can have just as much fun on a little rapid as a big one, and it's much safer," he said.

After meeting with riverboat company representatives and other government officials today, park Superintendent Richard W. Marks lifted his two-day-old suspension of river trips, but left in force a requirement that boat guides have passengers hike around the most dangerous point of the Crystal rapids.

Two weeks ago, Marks wrote to Clifford Barrett, upper Colorado regional director of the U.S. Bureau of Reclamation, asking that he be kept informed of additional water releases from Glen Canyon Dam that had "the potential of significantly impacting river trips."

Hodapp said that fortunately the flooding came at a time when Bell's vireo fledglings had left their nests and could seek refuge in foliage higher up the canyon wall. He said there might be more serious damage to the canyon bottom, however, if the Bureau of Reclamation, which controls Glen Canyon Dam, is not careful to lower the river flows gradually once the flooding danger eases.