[env-trinity] CBB: Study Looks At How Elwha Dam Removals Changed Nearshore Ecosystems Near Mouth

[Sari Sommarstrom]

The Columbia Basin Bulletin: Weekly Fish and Wildlife News
www.cbbulletin.com      January 19, 2018               Issue No. 859

 

Study Looks At How Elwha Dam Removals Changed Nearshore Ecosystems Near
Mouth

 

In the time since the removal of two dams on Washington's Elwha River,
scientists from University of Washington-based Washington Sea Grant, the
U.S. Geological Survey, Washington Department of Natural Resources, the
Lower Elwha Klallam Tribe, the Environmental Protection Agency and the UW
have sifted through eight years of data collected before and after the dam
removal projects to analyze the impacts the resulting sediment load has had
on the nearshore ecosystems near the mouth of the Elwha River.

 

The dams sat on the river for more than 100 years, trapping approximately 30
million tons of sediment behind their concrete walls. As the dams were
removed between 2012 and 2014, much of this sediment was released
downstream.

 

"The main impetus for the dam removal was the salmon reintroduction," said
study co-author Stephen Rubin, a USGS fishery biologist. "But it was also
about a whole ecosystem restoration, all the way from above the dams to the
strait."

 

In the short-term, however, some hypothesized that the sediment influx to
the nearby coastal marine ecosystem could negatively impact certain species.
This study sought to monitor these potential impacts and the ecosystem's
progression as it adjusted to post-dam life.

 

They found that how organisms were affected depended on the type of sediment
that was deposited in their habitat (for example, whether it was sand or
mud) and the local turbidity (how murky suspended sediment had made the
water). Still, some organisms were more resilient to the changes than
others.

 

According to the study, the dam removal projects did not result in
significant overall changes in the invertebrate or fish communities. Kelp,
however, markedly decreased. Understandably so, since kelp relies on light
for photosynthesis, and when more sediment is suspended in the water, there
is less light available to them.

 

"We viewed this as an opportunity to find out what really happens to the
nearshore environment after a dam is removed," said co-author Ian Miller,
coastal hazards specialist at Washington Sea Grant.

 

Their findings
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0187742
were published in the journal PLOS ONE in December 2017.

 

The researchers used SCUBA surveys and towed video transects to quantify the
relative abundances of algae, invertebrate species and benthic fish living
along the shore near the river, and then compared these findings to the
patterns of sediment influx using data collected by the USGS and the UW
School of Oceanography on substrate changes, and MODIS satellite imagery to
track changes in the suspended sediment in the water column.

 

"We had a huge suite of data we were working with, which is one of the
things that made this such a complicated endeavor," Miller said.

 

While the study showed a decrease in kelp, Miller notes that these are still
relatively early days on the Elwha's road to recovery. More recent data
suggests that, over the last two years, kelp populations at the mouth of the
river have already begun to rebound.

 

"Even where we saw a negative consequence for one group of organisms, kelp,
in our more recent surveys we've already begun to see a rapid recovery,"
Miller said. Another proverbial grain of sand to add to the pile of
knowledge that could help resource managers elsewhere assess the impacts of
events that send large amounts of sediment into the coastal zone - and
answer questions such as whether to remove dams on their own local rivers.

 

Abstract

The coastal marine ecosystem near the Elwha River was altered by a massive
sediment influx-over 10 million tonnes-during the staged three-year removal
of two hydropower dams. We used time series of bathymetry, substrate grain
size, remotely sensed turbidity, scuba dive surveys, and towed video
observations collected before and during dam removal to assess responses of
the nearshore subtidal community (3 m to 17 m depth). Biological changes
were primarily driven by sediment deposition and elevated suspended sediment
concentrations. Macroalgae, predominantly kelp and foliose red algae, were
abundant before dam removal with combined cover levels greater than 50%.
Where persistent sediment deposits formed, macroalgae decreased greatly or
were eliminated. In areas lacking deposition, macroalgae cover decreased
inversely to suspended sediment concentration, suggesting impacts from light
reduction or scour. Densities of most invertebrate and fish taxa decreased
in areas with persistent sediment deposition; however, bivalve densities
increased where mud deposited over sand, and flatfish and Pacific sand lance
densities increased where sand deposited over gravel. In areas without
sediment deposition, most invertebrate and fish taxa were unaffected by
increased suspended sediment or the loss of algae cover associated with it;
however, densities of tubeworms and flatfish, and primary cover of sessile
invertebrates increased suggesting benefits of increased particulate matter
or relaxed competition with macroalgae for space. As dam removal neared
completion, we saw evidence of macroalgal recovery that likely owed to water
column clearing, indicating that long-term recovery from dam removal effects
may be starting. Our results are relevant to future dam removal projects in
coastal areas and more generally to understanding effects of increased
sedimentation on nearshore subtidal benthic communities.

 

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