Why it’s important to let sediment reach the shore

In the Benediction for his 1968 autobiographical book, Desert Solitaire: A Season in the Wilderness, American author, essayist and environmental advocate Edward Abbey wrote, “May your rivers flow without end, meandering through pastoral valleys.” Today, many of those pastoral valleys have become homes to dams — altering the flow of rivers, sometimes ending it prematurely, and creating issues downstream we have only begun to understand. In June 2022, Evan Dethier, then a postdoctoral fellow at Dartmouth, led a study using satellite images from the NASA/USGS Landsat Program and digital hydrologic data records from 1984 to 2020 that found that dam building in the 20th century had caused the rivers of North America, Europe/Eurasia and Asia to deliver 49% less suspended sediment to the oceans. “Humans have been able to alter the world’s biggest rivers at rates that are unprecedented in the recent geological record,” Dethier said in a release of the study’s findings. In 2019, hydropower, including that generated by dams, accounted for an estimated 60% of all renewable electricity. Dams are important for preventing floods, and they provide water for domestic and industrial use; it has been estimated that as much as 40% of irrigated land globally relies on dams. The Hoover Dam generates electricity for 1.3 million people in California, Nevada and Arizona; provides water for 16 million people; and irrigates more than 1.5 million acres (600,000 hectares) of land. But, says Dethier, those benefits come with downsides. “Dams alter the seasonal passage of water,” he says. “They also block the passage of fish and other important organisms. And, as we continue to learn, they trap vast amounts of sediment.” To understand why that’s important, we need to take a closer look at sediment. Biodiversity Bed A 2011 estimate put the amount of sediment trapped behind the world’s dams at 73 cubic kilometers (18 cubic miles) — more than two times the maximum storage capacity of Lake Mead, the reservoir formed by the Hoover Dam and the largest reservoir by volume in the U.S. It’s estimated that by the middle of this century, dams worldwide will lose about… Read More
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