Up and down the Oregon coast instream barriers impede the migration of both juvenile and adult coho. Physical barriers come in many forms, including low head dams, push up dams, culverts, water diversions, tide gates, and levees. These barriers limit the ability of juvenile coho to access off-channel areas that provide refuge from high flows, as well as sources of cold water that provide refuge from high summer water temperatures. These barriers can also limit juveniles’ ability to access tidal areas within a tolerable range of salinity. Access to a wide salinity gradient is essential to coho as they undergo “smoltification,” a physiological change that juvenile salmon undergo as they transition from freshwater to salt. In addition to limiting juvenile access to high quality rearing habitats, barriers also limit the amount of spawning habitat available to adults.
By eliminating fish passage barriers, we can support both the juvenile and adult life stages of coho, while also expanding the distribution and types of habitats available to them. A well distributed mosaic of diverse habitat types throughout the watershed promotes life history diversity within a coho population. This diversity is important to ensure a population has sufficient resilience to adapt to changing watershed conditions resulting from climate change and other environmental variability.