Which of the following is the primary challenge that dams present to salmon?

Dams can block the paths that salmon must take to get to spawning grounds and can change the environment drastically, which can greatly effect animals in that environment. There habitats are altered from this result as well. Dams can also cause water temperatures to rise, and can reduce the numbers of spawning grounds, which effects that species. Sorry if this is all wrong, but I hope it helps