What type of stream is this? Where does it come from and where does it go? Pretty much any fish in a stram has the potential to be migratory, as others have said. The only reason I see to think the fish would disappear would be if this is an extrordinarily small stream that only received fish fish from a flooded source somewhere up stream. I don't have much experience with warmwater streams but it's amazing the conditions that native brook trout can live through and they're one of the most sensitive fish I know of. During the summers some of the streams around here dry up to just a few pools and rocksbut the fish survive, and conditions are equally if not more brutal in winter if it's cold and dry, but they survive. Can it put a dent in populations? Yes, but nature has a way of working things out if people don't get too much in the way. If these fish are a population that's been there a long time, they'll probably make it. One thing you could do if you're super concerned is build some small dams out of rocks and or logs to deepen the holes the fish ore most often found in.
