No doubt, you are sitting somewhere right now, reading this paragraph, in a watershed of some sorts. This matters alot to our friends the wild salmon, and their relatives. Helpful people are trying to prevent things like oil spill runoff, antifreeze, factory waste, untreated wastewater, etc. The following links will help you to find out more about the water and salmon protection groups in your area. I forgot to mention dams.
Dams make some big changes to watersheds.
They make it hard or impossible for fish to get upstream. Many dams have proven unnecessary, and are being removed. Others are being considered for removeal. Be sure to check out removedams.org for more info about dam removal in the Cascadia Bioregion. That's another story.