Gold Rush Alaska

Sorry, I ment to tie in the part about dredging. The dredges will turn up all kinds of silt and heavy metals that will get into the water and affect not only how well the fish can breathe, but also affect the biologically.

There is an old ghost town in Idaho called Silver City. It used to be a huge mining town. Some of the mines up there ended up leaking toxic heavy metals into the watershed. If you catch some of the trout from the streams up there ( which you are warned not to eat due to the toxicity ) you can find traces of the heavy metals in them and see how it affected them ( stunted growth, immature reproductive organs etc. ) when you compare them to the same species of trout but from an unaffected water shed.