The Colorado River backs up behind the Hoover Dam.


Rock History T e dams have changed the river’s fl ow. Less water rushes through the canyon today. Yet raſt ing down the river is still a thrill. Steep and narrow rapids make parts of the trip feel like a roller coaster ride. Waterman paddles past steep canyon


walls. T ey rise up high. He sees layers of colored rock. Each layer, or strata, came from a diff erent time in Earth’s history. T e rock near the water’s edge formed two billion years ago. He looks up. T e rock at the top of the cliff s is much younger. It only formed 265 million years ago. “Your raſt is like a time machine,”


Waterman says. “It takes you back millions of years.” T e rocks remind Waterman that the Colorado’s story began long before people, dams, and cities.


22 NATIONAL GEOGRAPHIC EXTREME EXPLORER


Red River Dams have changed the river’s ecosystems, too. Only the deepest water fl ows through dams. T e water is so deep, the sun can’t warm it. So the river is colder now. Some fish species can’t survive the chill. T ey are dying out. T ere are other problems, too. Moving water once spread silt along


the riverbanks. T ese muddy bits of soil, sand, and rock gave nutrients to plants and animals. Silt also turned the river red. Today, dams trap much of the silt. It’s stuck at the bottom of lakes. T e water is clear. It’s pretty, but not as good for the environment. Climate change aff ects the river, too.


Rising temperatures means less snow falls. T at shrinks streams that feed the river. It lowers water levels. All these changes mean less water and food for plants and animals.


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