Because dams are built to store water, they also store the sediment that all rivers carry. This sediment builds and steadily decreases the storage capacity of the reservoir. Ultimately all dams fill with sediment or are destroyed by natural floods.

Built in 1963, Glen Canyon Dam is 563 feet high and has steadily been filling with the equivalent of 30,000 dump truck loads of sediment every single day—100 million tons of sediment annually.

The 200-foot high Matilija Dam (left, photo courtesy of Matilija Coalition), has completely filled in with sediment in only thirty years. It has been decommissioned and the process of removing the dam and restoring the river has begun. Because the reservoir behind Matilija Dam has filled with sediment, the costs of decommissioning this dam are very high. If the decision to decommission the Matilija Dam had been made earlier, the restoration costs would have been greatly minimized.

Glen Canyon Dam will share the same fate as the Matilija Dam. It too is temporary, and the longer we wait to address the temporary nature and ultimate risk of Glen Canyon Dam, the more expensive it will become. The true and staggering costs of sedimentation will ultimately be born by the next generation unless practical solutions are developed now.

Sediment Health Concerns

Natural and human caused levels of heavy metals and toxins are combining in the reservoir. Arsenic, lead, selenium, boron, and mercury from upstream sources are currently trapped in the sediments of the reservoir, rather than flowing harmlessly to the sea, as they did prior to the dam. In addition, the flooding of Glen Canyon covered a yellowcake uranium mill tailings pile near Hite. The water percolating through this toxic sediment may pose health risks to fish, wildlife, and humans who visit Lake Powell. With lower water levels at Lake Powell, heavy metal concentrations are become higher.

Aggradation

Aggradation is defined as the accumulation of sediment where the river flow slows; in this case, as it approaches Lake Powell. Aggraded sediment deposits accumulate upstream of the reservoir elevation, eventually piling up hundreds of feet above the current reservoir level and burying many more miles of the Colorado River and its tributaries. As the river migrates across and through the deltaic sediments, it seeks a new level and position. It is impossible to predict where the river will migrate to due to the variable levels of flow, the sediment composition, and the solidity of the delta deposits.

Skeptics claim that sediment burying Glen Canyon might take a century or more to clear out and fully reveal the canyons. The dropping water levels experienced in the past 10 years, especially in 2005, have proven this to be false. In reality, restoration of the side and main channel canyons is remarkably rapid, as seasonal monsoonal and spring floods wash out huge deposits of sediment.

In the 1980s, high water silted up this side canyon on the lower Escalante (far left, 1983). As the reservoir dropped over the next few years, summer rainstorms caused flash floods that moved out the sediment (left, 1993), returning the canyon to its original state. Riparian habitat quickly returned to the canyon and the reservoir deposited mineral “bathtub ring” faded within several years.

In 1999, this beach (right) at Imperial Rapids, in Cataract Canyon, was covered with 50 feet of sediment from Lake Powell. The river’s natural flows have since restored the canyon.

Scientific studies predict that without the reservoir, sediment deposits in the main channel upstream of the dam could be flushed out in as little as five years (CEA). The actual time is dependent on the future hydrologic events occurring in the Colorado River Basin. As sediment builds up in Lake Powell reservoir, water storage capacity steadily decreases, and will ultimately fill the reservoir completely.