zh Kent-
Nice idea, an anti backflow valve would come in handy with that set-up. What is a coffer damn? With your described layout, I'm not sure if the silty water during a peak run-off event is enough to worry about. Ideally, no silt is the way to go, wich is why I was wondering if an automatic flow valve existed. The stream some times dries up during the summer, but I've seen the same stream over flow the road and a 2 foot culvert pipe. Duringing that event the contractor that has the pond said the stream spilled over the inlet to the pond as well as flowed over the dike (piped outlet).
Thanks, Chris