Biological Impacts of the Elwha River Dams and Potential Salmonid Responses to Dam Removal

The researchers in this article discuss the biological implications of the Elwha River dams for 6 salmnoid species and the potential benefits of dam removal. These dams are responsible for a 94-year disconnect between the upper and lower reaches of the Elwha, which has disrupted salmon access to spawning grounds and reduced their overall freshwater habitat by 90%. This has led to the loss of many populations of these species and dramatically decreased population sizes from what is estimated for historical, pre-dam species numbers. 

These researchers theorize that dam removal will increase salmnoid carrying capacity of the Elwha rivershed by reconnecting these animals with upstream habitats and traditional spawning grounds. Part of the potential benefits of eliminating this barrier are facilitated by restoring the downstream flow of sediment and woody debris leading to long-term aquatic ecosystem nutrient cycling improvements. GIS modeling of the ecosystem with and without the Elwha River dams allowed these scientists to predict salmnoid responses to dam removal. These researchers anticipate that after dam removal the 6 salmnoid species would establish hardy, self-sustaining populations in the upper reaches of the Elwha within one to two generations.

The researchers identified several key aspects of the 6 salmnoid species ecologies that are expected to affect recolonization ability after dam removal. The various metrics used to determine recolonization ability on a species-specific basis include: "barriers to migration, distance from a source population, source population size, stray rates, the amount, quality, and type of habitat in relation to specific life stage needs, and interactions with existing fish populations (Pess et al. 2008)." These measures were applied to construct intrinsic potential maps for post-dam removal distribution of each of the 6 salmnoid species in the Elwha rivershed.

Pess, G. R., McHenry, M. L., Beechie, T. J., & Davies, J. (2008). Biological impacts of the Elwha River dams and potential salmonid responses to dam removal. Northwest Science, 82(1),72-90.