I think it sounds like a neat project, but would likely be a futile effort without pretty intense construction and planning. If your floods, it would probably fill back in very quickly with sand and rocks. On top of that, if the water rises, I would bet your fish will leave. When I was a kid I would try building homemade swimming holes. It would only last until the next rain. I think you would have better luck going beaver style and damming the water downstream. Probably illegal, but I bet it would be pretty sweet for recreation.
