February 22, 2019

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 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.