At one point, 50% of salmon returning to the Columbia River Basin spawned in the Snake River or its tributaries. Today, only 1% of these salmon return to their spawning grounds as adults to reproduce. Each interaction a juvenile salmon has with a dam reduces their chance of returning to the Columbia River as an adult by 10%. Snake River salmon have to pass eight dams during migration: four on the lower Snake River and four on the mainstem Columbia River, bringing their chance of returning as adults to just 20%. Removing the lower Snake River dams will remove half of the major obstacles blocking fish migration to and from their spawning grounds.