"Does Dam Removal Work? An Assessment of Dam Removal As a Tool to Restore Native Anadromous Pacific Salmonids"

Dam removal is an increasingly common restoration choice for degraded rivers, though can be expensive and sometimes controversial. Most removals include the goal of reconnecting upstream habitat to benefit downstream species, but there is little evidence of dam removal’s impacts on fishes’ movement into this newly connected habitat. I have compiled available data on upstream re-colonization from all dam removals intended to restore anadromous salmonids (Oncorhynchus spp.) in California, Oregon, and Washington since 1999. I found that in cases where dams completely blocked upstream passage, anadromous salmonids were again present upstream of the former dam site in nine cases, absent in two, and uncertain in three. I have found that in most cases where dams had partially blocked upstream passage, monitoring was too inconsistent to show changes from the dam removal. In four cases, dams were removed despite the presence of downstream barriers. I conclude that dam removal is largely effective, with important differences between cases. I further conclude that in an era of limited resources, more consistent and affordable monitoring systems should be implemented as part of dam removal and other river restoration projects. I offer a monitoring framework to apply to future removals, which could offer consistent and informative data without demanding significant resources.