Improving connectivity between freshwater and marine environments for salmon migrating through the lower Snake and Columbia River hydropower system

Snake River stream-type Chinook salmon smolts migrate >1000 km from rearing habitats to the Pacific Ocean and return 1–3 years later for their upstream spawning migration. Construction of 8 mainstem dams on the Snake/Columbia River that fish must pass has greatly altered the connectivity between their freshwater spawning and rearing habitats and the ocean. In addition to direct mortality to smolts passing through turbines, these dams along with over 200 additional dams and storage reservoirs above them affect the volume, timing and turbidity of river flows and the size of the plume entering the ocean. At mainstem dams, improvements to fish ladders have largely eliminated problems for upstream migrants, while construction of screened bypass systems, a spill program, and transport of smolts by barge have greatly improved direct survival of juveniles. However, smolt-to-adult returns have not shown the same improvement and have been highly variable in recent years. While direct survival for juveniles passing 8 dams is now as high or higher than historically when they passed only 4, survival downstream of the last dam has changed in part due to altered timing and condition of smolts upon ocean entry. Recent additions of surface passage structures at dams have reduced travel time through the system to more closely approach historical rates prior to dam construction. However, substantial additional improvement in direct survival of smolts through the hydropower system does not appear achievable with existing knowledge and technology. Restoring conditions in the plume might improve survival but would require increased flow volume.