[env-trinity] CBB: Researchers synthesize dam removal studies

Fri Feb 22 15:04:21 PST 2019

Columbia Basin Bulletin

Researchers Synthesize Dam Removal Studies; Suggest Ecosystem Effects Can Be
Predicted 
Posted on Friday, February 22, 2019 (PST) 

In the United States, the removal of dams now outpaces the construction of
new ones--with more than 1,400 dams decommissioned since the 1970s--and a
new study suggests that the ecosystem effects of dam removal can be
predicted.

Published in the journal BioScience, the study
<https://academic.oup.com/bioscience/article/69/1/26/5285462>
https://academic.oup.com/bioscience/article/69/1/26/5285462 identifies a
consistent set of physical and biological processes that control ecological
responses to dam removal. These processes, combined with the unique
environmental conditions found at each dam, ultimately determine how the
ecology of the river will respond.

"We found that each dam removal is unique because of the location, size, and
history of the watershed," said Ryan Bellmore, a Juneau, Alaska-based
research fish ecologist with the U.S. Forest Service's Pacific Northwest
Research Station who is lead author of the study. "Nevertheless, we found
that ecological responses generally follow similar patterns."

Although dams are primarily removed because they are aging and are costly to
repair or upgrade, ecosystem recovery also is a common objective,
particularly for fish species such as salmon,

Bellmore and 14 other government, university, and nonprofit organization
colleagues synthesized more than 125 dam removal studies and ecological
theory into conceptual models. 

These models reveal the key physical and biological factors responsible for
driving ecological responses to dam removal and show that these factors--and
associated ecological responses--are distinctly different upstream and
downstream of removed dams.

Upstream of former dams, the major driver involves aquatic species
recolonizing habitats that they were not able to access with the dam in
place.

Downstream of former dams, the river often receives a surge of sediment that
had accumulated in the former reservoir, which can lead to major--although
temporary--effects on aquatic organisms.

Finally, in the former reservoir itself, species that are adapted to slower,
deeper water are replaced by those adapted to shallower, faster moving
water.

"One of the desired outcomes of dam decommissioning and removal is the
recovery of aquatic and riparian ecosystems. To investigate this common
objective, we synthesized information from empirical studies and ecological
theory into conceptual models that depict key physical and biological links
driving ecological responses to removing dams," says the study abstract.

"We define models for three distinct spatial domains: upstream of the former
reservoir, within the reservoir, and downstream of the removed dam. Emerging
from these models are response trajectories that clarify potential pathways
of ecological transitions in each domain. We illustrate that the responses
are controlled by multiple causal pathways and feedback loops among physical
and biological components of the ecosystem, creating recovery trajectories
that are dynamic and nonlinear. In most cases, short-term effects are
typically followed by longer-term responses that bring ecosystems to new and
frequently predictable ecological condition, which may or may not be similar
to what existed prior to impoundment."

Managers and dam-removal practitioners can use the study's models to gauge
the potential range of ecological responses to dam removal and the most
likely future conditions, helping generate more realistic expectations for
ecological recovery.

"This research expands our conceptual understanding and improves our ability
to predict response to future dam removals," said Jill Baron, co-director of
the U.S. Geological Survey's John Wesley Powell Center for Analysis and
Synthesis, which sponsored the work.

The study was conducted by the Dam Removal Synthesis Working Group, a team
of 22 scientists from the U.S. Forest Service, U.S. Geological Survey, NOAA
Fisheries, Oregon State University, University of Montana, Dartmouth
College, Bowling Green State University, and American Rivers.

The Pacific Northwest Research Station--headquartered in Portland,
Ore.--generates and communicates scientific knowledge that helps people make
informed choices about natural resources and the environment. The station
has 11 laboratories and centers located in Alaska, Washington, and Oregon
and about 300 employees. Learn more online at
<https://www.fs.usda.gov/pnw/> https://www.fs.usda.gov/pnw/.

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