In the American west there is no hard and fast date for when runoff from snow melt is over. It varies from year to year based on the winter’s snow water equivalent, and from location to location due to regional climatic differences. Of course the best way to know the optimum time for summer fly fishing is to have some years of experience and to have either a written or mental logbook of when runoff is over and the rivers have reached their fishing peak.
But what about trying to get your timing right in an area you have no experience with? Here is a semi-scientific way, based on USGS long-term hydrographs of estimating the best time. A quick refresher on runoff and stream flow http://streamsideadventures.com/tips-and-techniques/stream-flow-and-fly-fishing-runoff-hydrographs.html and also how a stream gauge works http://streamsideadventures.com/tips-and-techniques/stream-flow-and-fly-fishing.html will help you with making your estimate.
One of the things you will note when you look at a snow melt stream’s hydrograph is that at the start of summer you see the “peak flow” and late in the summer the stream
flow reaches a rather consistent level, what hydrologists call the “base flow.” Some place in between these two points is the best time to fish. But when?
As you look at the hydrograph you will generally notice a point in time when the slope of the hydrograph makes a bit of a change. A good way to find this is to draw a line, starting at the peak, along the points on the hydrograph.
The place where the individual data points veer away from your line is a very good estimate of when the major period of runoff is over! Sometime around that date – plus or minus a about a week – is your prime fishing time.