In the case of the salmon, the dams along the Columbia River have had a huge impact on the fish's ability to travel up- and down-river to and from the spawning grounds. Fish ladders help, but they still prevent many fish from passing. Then, on the way back down, the young molt have about 2 weeks to make it to the ocean, which is when their biology shifts from fresh water to salt water. But the dams have transformed the river into a series of slow moving lakes that can't carry the molt as fast as they need to travel. The Corps of Engineers has actually begun trucking salmon molt downstream to the ocean so they can make it in time.
My grandfather, who grew up working on riverboats on the Columbia, says he can remember when the salmon return was so plentiful you could almost walk across the river on their backs.
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My grandfather, who grew up working on riverboats on the Columbia, says he can remember when the salmon return was so plentiful you could almost walk across the river on their backs.
Wikipedia as a list of the other environmental pressures on salmon:
http://en.wikipedia.org/wiki/Salmon#Environmental_pressures
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