Gold Rush Alaska
Good question! Maybe someone here will chime in with an answer?
I played around a little bit in the rivers here in Colorado and didn't even find a flake. :-D
I spent 3 years in college writing papers and doing studies regarding the salmon population and the effects of the dams along the Columbia and Snake River, this is where my knowledge comes from. While it may not be exactly the same for other areas, the general principles still apply.
The biggest thing that hurts the salmon in terms of the dams is the water getting cloudy and slow. Biologically, when the salmon are still young, they instinctively know to swim with the current and to reach the ocean. When they are older, and starting to migrate back to their spawning grounds, they instinctively know to swim against the current. What happens when the rivers are dammed up, is the current almost becomes non-existent ( think in terms of what the current would have been 500 years ago ) confusing the salmon.
Salmon also need clean, clear cold water to thrive in. Water that is dammed up and slowed down also warms up making it not very suitable for the salmon. The food niches that the salmon would ususally feed on ( in fast flowing water ) are not there in the slow lethargic water due to dams, decreasing drastically what they can eat.
But, before you get all gun ho about tearing down the dams, you need to realize that doing so will KILL more salmon that leaving them in place. You have 50+ years of silt built up behind these damns along the Columbia. When you tear down the dams, the silt will flow down the river choaking out all of the organisms living in the river. The most logical time to tear down the dams would be during late summer/early fall while the water flow is the lowest. This time also corresponds to some of the more endangered salmon runs ( at least for the Columbia Basin Drainage ). So, by tearing down the dams to save the salmon, you will actually be killing them and driving them ever closer to extinction.
RTom