Dams, Sediment, and Habitat

This week, the reservoir (Lake Mills) behind Glines Canyon Dam on the Elwha River completed draining.  While there's still about 50 feet of the dam standing, the river creates a waterfall over what's left of the dam.  However, now that the reservoir has been drained, the Elwha River has been pushing the sediment that has been trapped behind the dam since it was built in 1926 downriver and out into the Strait of Juan de Fuca.  There are both benefits and impacts of this release on sediment on the down stream habitats and ecosystems, but the ecological benefits definitely outweigh the short-term impacts.

After a controlled blast drained the remaining Lake Mills, which restored the the flow in the Elwha River.
Courtesy of the National Park Serice

The beginning of the fall and winter rains has swollen the river and has dramatically increased the amount of sediment flowing downstream.  Over 24 million cubic yards of sediment had settled behind Glines Canyon Dam and Elwha Dam and "thirty to fifty percent of this sediment is expected to erode downstream over the next three to six years."  Scientists predict that during November and December alone sand that had accumulated over 20 years and gravel and cobble that had accumulated over 5 years will be released downstream.

The sediment-filled Elwha River.
Courtesy of The Seattle Times.

Turbidity, a measure of the amount of sediment suspended in cloudy water, is one way hydrologists are monitoring the sediment released from behind the dams.  Turbidity in the Elwha River has increased more than seven fold since the summer.  This increase is precisely what the scientists, engineers, and managers had predicted and hoped for.  With the increasing rains and higher river flows, the river is moving large amounts of clay, silt, sand, coarser sediment (like gravel) and woody debris downstream.  

Sediment released from behind dams can have both benefits and impacts on the downstream ecosystems.  When dams are put in, they block a lot of sediment from flowing downstream that had previously had done so.  Much of the sediment is now deposited behind the dam and alters the river's habitat.  Because much of the sediment is deposited behind the dam, the water that passes through is "sediment starved" and often regains sediments by eroding deeper into the stream bed and banks.  This significantly alters the habitat downstream of the dams and can have serious implications for ecosystems.  Furthermore, sediment trapped behind dams can accelerate the loss of coastal shoreline habitats because rivers transport much of the sediments that comprise these habitats.

A beach reforms along the Elwha River, downstream
from the Glines Canyon Dam.
Courtesy of the National Park Service

Removing dams can reverse many of these adverse impacts of trapping sediments behind dams.  It can restore natural river sedimentation patterns.  Often, gravel and cobble upstream of the dam are exposed when the reservoir is drained and this can create new habitat for colonization by insects and revitalize spawning grounds for fish.  Furthermore, dams can be key in restoring sediment to coastal beaches/shoreline.  On the Elwha River, the dams have trapped many of the sediments important for creating shoreline beaches, which has resulted in beach erosion and has allowed non-native species such as kelp and barnacles to become prolific in the Strait of Juan de Fuca.  Additionally, the trapping of sediment by the dams has caused loss of critical estuaries.  However, studies predict that removing these dams will reverse these negative impacts.

Sediment from the Elwha River in the Strait of Juan
de Fuca.
Courtesy of USGS.

There are, however, short-term ecological impacts of sediment released when dams are removed.  The increase in turbidity and decrease in water quality can damage spawning grounds, and negatively impact water, habitat and food quality.  Additionally, the increase in turbidity can reduce the amount of light in the river and can inhibit aquatic plant growth and have impacts on wildlife that must see to find food and avoid predation.  Fortunately, these impacts are usually fairly temporary and can be mitigated to some extent.  For example, on the Elwha River, all work that would release sediment has been stopped for November and December in order to create a "fish window" to help protect chum and coho salmon returning to the river to spawn.

The sediment released from behind the dams on the Elwha River are having likely having effects on salmon currently.  While the river is too cloudy to tell if they are being killed by the sediment, they are heading to clearer, cleaner waters in other tributaries, the tribal fish hatchery, and the state rearing channel.  While a few nests have been found upstream, scientists agree that the sediment levels are generally too high.  However, more than 300 coho have found their way to the tribal hatchery (located on the lower sections of the river), which is enough to sustain the population.

Sediment flows out of the Elwha River and into the Strait of Juan de Fuca.
Courtesy of the Peninsula Daily News.

Despite these short-term impacts on the salmon, the long-term benefits of dam removal, which will allow them to spawn in the upper reaches of the river that have been blocked for decades, clearly outweigh them, especially because the salmon are finding other options for spawning in the meantime.