Perhaps the most definitive barrier to fish migration and spawning is an actual barrier. Hydroelectric dams are beneficial to human development, but devastating to fish populations. Many dams have been built with zero allowances for fish passage. This completely cuts off all spawning and rearing habitat above the dam and changes the natural flow and substrate distribution of a river.
Some dams upstream in the Columbia River did not have fish passage. This destroyed populations such as the famous "June-Hog" Chinook Salmon that migrated to the upper reaches of the Columbia. These fish were genetically built and adapted so that they could migrate large distances to reach their spawning grounds. This resulted