Featured image: A sandbar along the Colorado River in the Grand Canyon. Photo by the author. CC-BY-SA.
Authors: Erich R. Mueller and Paul E. Grams
Journal: Geophysical Research Letters
The Grand Canyon is famous for its stark, bare-bedrock landscapes. But those who make the hike, mule ride, or raft trip into its depths are rewarded with a different view: the green, inviting banks of the Colorado River where ancestral Puebloans once grew corn and where rafters now collect overturned boats between rapids. The Canyon owes its bucolic river-bottom landscape to an unsung hero: sand deposited during large floods that creates hospitable habitat for plants and aquatic animals. Since the building of Glen Canyon Dam just upstream in the 1950s, the Grand Canyon has been starved of sand, damaging its fragile ecosystems. Now, a new study quantifies how controlled floods could help restore sandbars to the Grand Canyon.
Glen Canyon Dam traps sediment in Lake Powell and reduces the size of the river’s annual floods. The absence of these sandbar-forming floods and the high sand loads they would naturally carry has shifted the Canyon’s sandbars into a regime of net erosion, in which low flows strip away sand that isn’t being replaced. The loss of sandbars in many reaches of the Canyon reduces the area suitable for vegetation and removes habitat for endangered fish and other aquatic life.
The effects of sand starvation in the Grand Canyon and other dammed rivers are well known, and have prompted attempts to mitigate the problem. Since 1996, the U.S. Bureau of Reclamation has released controlled floods from Glen Canyon Dam that are intended to help rebuild sandbars in the Grand Canyon. Releases are made when sand has built up along the Canyon from smaller floods on undammed tributary channels, and are intended to redistribute the sand into ecosystem-sustaining sandbars.
The idea of controlled floods to restore sandbar habitats is appealing, but many important questions remain. How much water should be released in the optimal controlled flood? How often should they be repeated? Which is more important, the supply of water or the supply of tributary-derived sediment? A new study by geomorphologists Erich Mueller and Paul Grams seeks to answer these questions.
Mueller and Grams created a computer model that simulates the building of sandbars as flood-borne sand is deposited in eddies, or zones of the river where water recirculates after flowing past the large debris deposits that form many of the Grand Canyon’s rapids. Their model also incorporates sandbar erosion, or the loss of sand volume over time during periods when sandbars are not replenished by large, sediment-laden floods. By combining their model with field measurements of flood size and sand concentration in the flood waters, Mueller and Grams were able to assess the conditions under which sandbars are most effectively built and preserved.
The authors confirmed using their model that large, sediment-laden controlled floods result in sandbars that are about twice as big as a case with no floods. Each simulated flood increased the volume of sand stored in sandbars by 30-100%, indicating that increasing the frequency of floods would also help restore sandbars given that bars tend to lose volume during low-flow periods between dam-release floods. By assessing the modeled response of sandbars to flood magnitude and sand concentration independently, the authors found that the two factors are approximately equally important in terms of their effect on sandbar volume. More floods, in short, would help restore the Canyon’s sandbars. If those floods are timed to coincide with sediment delivery from undammed tributary streams, even better.
Mueller and Grams have provided a new way to predict the effects of future controlled floods, and the predictions imply that restoration of the Grand Canyon’s sandbar habitats is possible. But water levels in Lake Powell are currently at a record low as a heat wave bakes the American West, and controlled floods take water from the lake that might otherwise be used to irrigate fields. The balance, if one exists, lies in managing the dwindling western water supply while ensuring that enough large controlled floods are released into the Grand Canyon to protect and restore the sandbars that have supported plants, animals, and humans for millenia.