Rivers split across mountaintops and other geographic barriers may flow only a few miles from one another, but to the aquatic creatures in those waters, the separation could represent millions of years of evolutionary time. So, when an angler or a curious child moves a fish from one side of the mountain or one side of the country to the other, it's a very big deal to the fish. Some may discover a competitive advantage in a new stream, potentially disrupting eons-old ecological hierarchies.