Rafting through time.

Join the thousands of "time travelers" who raft down the Colorado River through Grand Canyon National Park each year.

The water is splashing around you as you descend into a chasm with reddish walls--meters, 50 meters, 100 meters, 300 meters deep. Here the cliffs rise straight up, smooth as marble. Just beyond, they look like a stack of giant cards, layer upon layer rising high above you. Quickly, you've dropped nearly two kilometer's into the deepest gorge on the planet--no exit signs in sight.

No, you're not reaching level seven in some hip new video game. This trip is better than even the best virtual reality adventure. It's a real-life journey down the Colorado River to the bottom of Arizona's Grand Canyon--in a raft.

Your guides: river boatman Dave Foster, who first rafted down the Colorado in 1973 at age 13, and geology professor W. Ken Hamblin of Brigham Young University.

TIME OUT

"In a very real sense," says Hamblin, "this is a journey to the center of the Earth--although you don't make it quite to the center." As you ride the river downhill through Earth's rocky crust, you descend through layers of rock. The relatively young rocks, closest to the surface, hold fossils of 65-million-year-old dinosaurs. Below lie layers dating back nearly 2 billion years, to a time when single-cell sea critters were the only living things on Earth.

How did the Grand Canyon get this layered look? Way back before the canyon existed, Foster explains, an ancient sea covered the western part of North America. For millions of years, sand and the shells of tiny ocean organisms settled to the bottom. Pressure from the water and the layers of more debris that landed above gradually compressed and hardened these sediments. The result: layers of sedimentary rock--sandstone, shale, and limestone--stop much older bedrock, made of schist and granite, beneath this ancient sea.

Less than 100 million years ago, explains Ivo Lucchitto of the U.S. Geological Survey, for reasons scientists still don't fully understand,

the ancient seabed began to rise, forming a plateau on the western edge of the continent. Eventually the plateau reached 9,000 feet above sea level.

As a result of this uplift, the waters of the Colorado River, with its origins in the Rocky Mountains, cascaded steeply toward the Gulf of California. Flowing down that steep slope gave the water (and the sediments it carries) enormous energy--enough to cut a rift nearly two kilometers deep into the plateau.

As your raft carries you along, look over the side, urges river guide Foster. You'll see what does the actual canyon-cutting: a layer of coarse sediment made of mud, sand, and gravel covering the rocks on the river's bottom. As the river flows, this bottom sediment, called bedload, scrapes over the rocks like sandpaper over wood, slowly eroding (wearing away) the riverbed to carve the canyon.

SLOW TO A TRICKLE

The bedload only gets moving when severe thunderstorms or snowmelts swell the river. Though storms are still common, such swelling is rare on the Colorado today because, in 1963, engineers tamed the river. They erected Glen Canyon Dam to regulate the river's flow. That way they could capture some of the river's energy to generate electricity, and prevent massive floods. Because of the dam, today's Colorado is a trickle compared with its former roaring self.

But wait--what about that hair-raising white water you splashed through around the last bend? Foster explains: Tributaries, smaller streams that spill into the river, are not regulated by dams. When they flood, they dump huge loads of rocky debris into the Colorado. The tamed Colorado can't wash the rocks away. So the water backs up and crashes over and around the rocks, resulting in exhilarating, but dangerous, rapids.

Take Bedrock Rapids, says Foster. "It's a big rock in the middle of the river that's hard to get around," he explains. "And there's a side stream coming in on the right, bringing in more and more [rocky] debris .... You can't get around the debris ... and also get around the rock." The rapids have overturned many boats there, he says.

STILL, BUT NOT SILENT

In other places, such as Lake Powell, which formed behind Glen Canyon Dam, the river's flow has all but stopped, says Foster. Here, sediments settle out in the still waters. And the sun heats only the lake's surface. Result: colder, clearer water pours through the dam to continue the river's flow.

These conditions have given rise to a new river habitat, complete with cold-water-loving trout and bald eagles that eat them. That may sound ideal--at least for trout fishermen and eagles--but many native animals, such as squawfish and humpback chard, have lost most of their warm, muddy habitat and are now threatened or endangered.

Some conservation groups, notably the Grand Canyon Trust, are pressuring Glen Canyon Dam operators to adjust the Colorado's flow to create more-natural river conditions. But as long as the dam is in place, the conservationists say, the river will never be the same. The dam prevents the floods that were so important to the canyon environment.

Does that mean it's "Game Over" for the once mighty Colorado? Keep paddling and stay tuned.