Stuck sediment

The flowing course of a river carries with it more than just water – insects and other living things move downstream, too, and so do rocks, trees and branches, and sediment. When a dam is built, the fast-moving water of a river transforms into the slow-moving water of a reservoir, and stuff that might have moved downstream in the past can get stuck behind the dam.

River restoration and dam removal projects have proliferated in recent years, leaving scientists and resource managers wondering what happens to the multiple decades-worth of accumulated sand and mud that can built up at the bottom of a reservoir when the dam that formed it is suddenly gone.

When the two dams on Washington State’s Elwha River were demolished, more than ten million cubic meters of sediment were released into the river and allowed to flow downstream to the river’s estuary where it meets the Strait of Juan de Fuca. You’d need to rent more than two hundred and twenty-seven thousand 26-foot U-Haul moving trucks – the largest size they offer – to move that much sediment by truck.

A team of scientists, curious about how that much sediment would affect the Elwha River and its estuary, monitored two ‘pocket estuaries,’ small areas protected by barrier beaches but influenced by the tide, both before and after the dams were destroyed.

The researchers recently published the results of their study: the sediment rode downstream to the lower river, where it settled in the river channel and pushed the river delta more than 100 yards further into the sea, cutting the estuaries off from the influence of salt-water tides and filling them instead with water from the river, “changing the estuary from a brackish and tidally influences system to a perpetually freshwater system.”

When the river was dammed, water quality measurements (including salinity, depth, and temperature) varied according to the tides; after the dams were dismantled, they fluctuated in response to the amount of water flowing down the river. These physical changes to the estuary habitat have already altered the biology of the place – different insects live there now, and fish communities and diets are shifting as well.

The authors write that “[t]he removal of the [two] dams and the subsequent delivery and deposition of sediment to the river delta has caused the Elwha River system to lose its small, but important estuary habitat.” They also note, however, that “the potential for new estuary habitat to develop is high.”

Though the dams have come down, the Elwha River is still changing – and scientists plan to monitor the evolution of the river for years to come.

This photo was taken in 2012, during the dam removal project on the Elwha River. Sediment released by the demolition was deposited in and around the estuaries at the river mouth; some sediment also flowed into the Strait of Juan de Fuca as a coastal plume.

(Image by John Felis via USGS/Public domain)