Dave Wegner & Mary Gavan

Since before the gates at Glen Canyon Dam were closed in 1964, Glen Canyon has been capturing and releasing sediment from the Upper Colorado River Basin watershed to feed the beaches, the flood plain, and the aquatic and riparian habitats along the Colorado River.

While Glen Canyon Dam was authorized by Congress to provide the Upper Basin states with protection of their water from developers and farmers in the Lower Basin, it also provides the ability to control water releases to satisfy the Colorado River Compact and to generate hydroelectricity. As the dam slowed the waters and impounded Lake Powell, it trapped sediment — a lot of it — annually an estimated average of over 45 million tons of sediment.

Sediment forms the backbone and maintains the integrity of the beaches and riparian areas of the lower Glen and Grand canyons. Below the surface as that sediment flows downstream it creates temporary aquatic habitats important to the native fish populations and aids in the recycling of nutrients from the river into the ecosystem. Sediment, composed of eroded sandstone, shales, clays, and other materials is as varied as the geologic formations that it comes from. Since the early 1960’s the sediment coming into Lake Powell has flowed under the surface of the reservoir, cascading under the water towards Glen Canyon Dam. Today as the Lake Powell drops in elevation these sediment deposits are emerging along the main channel and in every tributary and side canyon that flows into the reservoir.

These exposed sediments tell a story. A story that involves the geology of the watershed, the management of the Colorado River, politics, ecosystem integrity, and the changes that we might anticipate as aridification and climate change settle over the Colorado River Basin. We are going to explore the sediment issues in Lake Powell and its implications to future management of water in Lake Powell and the Colorado River system and explore the relationship of sediment to the maintenance and sustainability of the water supply and ecosystem.

The story we will tell will focus around several questions:

· What is the geologic source of these sediments and where in the Upper Colorado River Basin do they come from?

· What is the composition of these sediments and do they have an impact on the water quality of the Colorado River?

· How much sediment flows into Lake Powell on an annual basis and how much variability exists between given years?

· How much of the Lake Powell reservoir capacity has been lost due to sediment? Which areas of the reservoir are most impacted?

· What are the implications for the tributary areas as Lake Powell drops in elevation and the inflowing rivers reestablish paths to the Colorado River?

· What is the prognosis for sediment flows into Lake Powell in a changing climate?

You are probably asking yourself — why should I care about sediment in Lake Powell? I cannot see it, it does not impact me directly (except when it gets in my sleeping bag or clothes), and certainly if the government does not care about sediment in Lake Powell — why should I?

These are valid questions and here is why it should matter to you.

1. As Lake Powell levels continue to diminish, the amount of water volume (capacity) available for release to the Lower Colorado River Basin is less than we thought — how much so is the question.

2. Sediment build up in the Colorado River and San Juan River arms of the reservoir are already having impacts in respect to flow dynamics of the incoming tributaries. The waterfall on the San Juan River below Clay Hills is a good example of a river establishing a new path to the Colorado River.

3. Continued trapping of sediment in Lake Powell is diminishing the size of the beaches and riparian areas in the lower Glen and all of the Grand Canyon. Beaches are getting smaller, riparian areas getting narrower, tribal cultural areas are threatened and archeological treasures are being exposed.

Glen Canyon Dam is a blip in the geologic record of time. Geologic history is replete with natural dams built by debris flows, lava dams, and shifting canyon walls. Sediment movement in Glen Canyon has had a natural variability due to changing weather and water runoff patterns and the exposure of erodible geologic formations. Over the course of the last 100 years research has shown that the Colorado River basin has witnessed variable sediment volumes as a result of hydroclimate changes and upstream geomorphic changes (development and building of additional upstream dams).

Reservoirs like Lake Powell are sustainable only as long as they offer sufficient water storage space to achieve their design objectives. While Powell’s primary reason for being is to provide water for meeting Colorado River Compact requirements, changes in hydrology will lead to changes in sediment which will ultimately impact the ability for Glen Canyon Dam to meet its objectives.