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)

Salmon eggs

Over 100 years ago, in 1910, workers began construction on the first of two hydroelectric dams that would eventually be built on the Elwha River, on Washington State’s Olympic Peninsula. Before the dams were built (the lowest just five miles from the river’s outlet on the Strait of Juan de Fuca), the Elwha was home to robust populations of several species of Pacific salmon. After the dams were built, most of the river was cut off from the ocean – salmon could no longer migrate back to freshwater to spawn, reproduce, and nourish the streams where they were born.

Last August, three years after the largest dam-removal project ever conducted in the U.S. began, the last section of the uppermost dam was demolished, and just weeks later, salmon were back in the upper Elwha.

Scientists anticipate that salmon will continue to follow their migrations upstream and recolonize the upper Elwha River; as they do so, some will encounter a mysterious population of fish living in Lake Sutherland, a small lake connected to the Elwha River by a creek that comes in above the location where the lower dam used to be.

Those mysterious fish are Oncorhynchus nerka, also known as sockeye salmon or kokanee. Sockeye salmon and kokanee are distinct populations of the same species that tend to either migrate to the ocean and return to freshwater streams to spawn, a trait scientists call ‘anadromy’ (sockeye salmon), or spend their entire lives in freshwater (kokanee). The population of Oncorhynchus nerka in Lake Sutherland was landlocked by the Elwha Dam for a hundred years, but researchers were not sure whether their ancestors were sockeye salmon trapped above the dam when it was built, or kokanee that might never have migrated to the ocean and back at all.

Now, a team of scientists believes they have the answer, and they came to their conclusion based on a humble clue – the size of the eggs the Lake Sutherland Oncorhynchus nerka produce.

As the researchers recently reported in the journal Ecological Research, they compared the Lake Sutherland fish eggs to eggs produced by several populations of sockeye salmon and kokanee from Alaska, Washington, British Columbia, and New Zealand. Kokanee eggs tend to be smaller than those of sockeye salmon (just as kokanee adults tend to be smaller than sockeye salmon adults). The Lake Sutherland fish themselves were typically about a foot long, the same size as the adults of the kokanee populations and half as big as the sockeye salmon adults.

Their eggs, however, were much larger than the kokanee eggs – and well within the range of the sockeye salmon eggs. The growth of the adult fish in Lake Sutherland appears to have been limited by their inability to access the ocean, and, based on the size of their eggs, it’s likely that the Oncorhynchus nerka in Lake Sutherland are descendants of sockeye salmon.

The scientists note that this has “immediate relevance to the restoration” of salmon in the Elwha River, because “it would mean that other traits linked to anadromy might also remain in the population, facilitating the resumption of anadromy in this population.”

Now that the dams on the Elwha River have come down, several species of salmon will once again be able to migrate upstream to spawn – and some Oncorhynchus nerka may finally be able to make it downstream to the ocean, before returning to freshwater to start the cycle anew.

The Elwha Dam in October 2011, about a month after the removal project began. 

(Image by Sam Beebe via Flickr/Creative Commons license)