Shihmen Reservoir releasing sediment through the power plant discharge outlet. Image Credit: Peellden
The accumulation of sediment in reservoirs across the American West represents a significant challenge for water resource management. Since the early 20th century, thousands of dams have been constructed to provide essential services, including hydro-electric power, irrigation for agriculture, and reliable municipal water supplies. However, these structures naturally trap the silt, sand, and clay carried by rivers.As these materials settle, they gradually affect the water storage capacity that is important for the survival of communities in arid regions. A major portion of global reservoir capacity is being lost annually, creating a long-term threat to infrastructure. In the western United States, where aging dams and shifting climate patterns are common, the management of this crisis has become a priority for federal agencies.
How big is the scale of dam sedimentation in the American West
According to the document titled ‘Sediment Bypassing – Preserving America’s Reservoirs,’ the United States has built and currently relies upon over 92,000 dams and reservoirs. These structures are vital for everything from municipal use to flood risk reduction, but their ability to function is being disturbed by the very rivers they built on.In the report ‘A Watershed Moment for Western U.S. Dams,’ researchers Amy E. East and Gordon E. Grant points out that since 1990, the amount of water storage capacity lost to sedimentation has outpaced the gains made by building new dams. When accounting for a growing population, water storage available per person in the United States has dropped by 35% since 1970. This suggests that the infrastructure growth of the last century is reaching a point where maintenance is more critical than new construction. As noted in the book chapter ‘Dams, Rivers, and the Environment,’ millions of dams have fragmented the Earth’s landscapes globally, and many are now permanent components of the environment that must be managed sustainably to prevent complete failure.
How sediment accumulation has been affecting the Lake Mead
The ‘2001 Lake Mead Sedimentation Survey’ produced by the US Bureau of Reclamation reveals that since the Hoover dam was completed in 1935, the reservoir has seen 2.4 million acre-feet of sediment deposition. To understand easily, an acre-foot is roughly the amount of water needed to cover a football pitch to a depth of one foot. Interestingly, the 2001 survey actually measured an increase in the reservoir’s capacity since the previous check in 1963.This phenomenon was not due to the removal of silt, but because of the closure of Glen Canyon Dam further upstream. As explained in the Bureau of Reclamation’s technical appendix ‘TA 5. Geomorphology and Sediment,’ the closure of Glen Canyon Dam in 1963 cut off approximately 95% of the historical sediment supply that used to flow into Lake Mead. The sediment already present in Lake Mead had nearly four decades to settle into a smaller volume, while very little new material arrived to take its place.
Sediment-laden water from the Colorado River flowing into Lake Mead in March 2013. Image Credit: NASA
What are the environmental costs of trapping river sediment
By trapping silt and sand, dams are cutting off downstream supply of sediments required for healthy ecosystems. ‘A Watershed Moment for Western U.S. Dams’ report notes that the lack of sand floods in the Grand Canyon has led to the erosion of sandbars and the exposure of irreplaceable archaeological sites that were once protected by wind-blown sand.Furthermore, the technical appendix highlights that because the Colorado River now transports sand downstream faster than tributaries can resupply it, there has been erosion of channel and sandbar deposits since 1963. This ‘sediment starvation’ simplifies the river’s shape and limits the formation of habitats necessary for native fish and wildlife. The trapping of sediments results in the lowering of river beds and the loss of wetlands, which are essential for flood-resistance and supporting biodiversity.
What strategies exist for future sediment maintenance
One strategy is ‘water injection dredging’ which is a cost-effective and environmentally sound technique. This method involves injecting large volumes of water into the sediment layer at low pressure to liquefy it, allowing the resulting mixture to flow along the bottom and remain within the natural river system instead of filling up a disposal site.Additionally, as structures become obsolete or unsafe due to excessive silt, dam removal is becoming a more frequent strategy. ‘A Watershed Moment for Western U.S. Dams’ report points to the historic removal project on the Klamath River as a sign of a changing paradigm in water management. For the remaining dams, strategies such as ‘sediment sluicing’(opening gates during flood events to let muddy water pass through) are being used in places like Asia and Europe to extend the life of these reservoirs.
Removal of the Marmot Dam, Sandy River, Oregon. Image Credit: National Oceanic and Atmospheric Administration(NOAA)
Climate change complicating the preservation of dams
As detailed in ‘A Watershed Moment for Western U.S. Dams’ increasing temperatures are leading to more frequent and intense wildfires. When a landscape is scorched by fire, the soil loses its grip and subsequent rains then wash massive amounts of debris and sediment into downstream reservoirs.In California alone, post-fire erosion has increased significantly, with over half of that sediment ending up in reservoirs. This creates a difficult cycle where reservoirs are filling fast, just when they are needed the most to manage the water supplies caused by a warming world. Go to Source
