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Science and Innovations

Earth Notes: Glen Canyon Dam - What Flows In (And Not Out)

National Park Service/Kyler Carpenter

Two hundred miles upstream from Glen Canyon Dam, the Colorado River roars through Cataract Canyon in a rust colored tumult, thick with silt and clay. Each year, the Colorado and its tributaries carry, on average, some 61 million cubic yards of sediment into Lake Powell, enough to fill more than 200,000 railroad boxcars.

When the Colorado meets the reservoir's still waters, it drops that load of suspended solids, forming a delta now 180 feet thick. Downstream, the San Juan River's own Lake Powell delta has mounded to a depth of 120 feet.

The reservoir's fluctuating water levels have caused sediments to drop out across a broad swath. As a result, deposits of new land now overspread 20 to 30 miles of both the San Juan and Colorado river corridors.

This sediment won't affect Glen Canyon Dam for a while, but it's already affecting how people experience the upper reaches of Lake Powell. As the lake level has dropped in the last 10 years, the San Juan and Colorado have carved new channels through these deposits. In places, sediment banks tower above the water, cutting off boaters' access to the shore.

In sections of the San Juan, the river fans across the sediment delta, inches deep, instead of flowing in a single, navigable channel. Upstream along the Colorado, sedimentation and low lake levels have rendered Hite Marina unusable. Maybe people in the Southwest can corral a river - but it's a lot harder to corral the erosion that made the river in the first place.