If there is one thing I have learned about fly fishing in the Rocky Mountain West, it is that nothing is constant. River levels rise and fall, insect hatches come and go, and a trout's mood can be influenced by any number of outside variables, both tangible and not. While it would seem logical that late summer would be one of the more consistent times of year to fish, the reality is that there are evolving circumstances that can change the make up of a river and how fish will subsequently react. Knowing how to identify these variables, as well as understanding their subsequent influence on the productivity of a stream, is a paramount skill that every fly angler in Colorado should have. So let's take a moment and discuss some of the key factors that influence the productivity of a river, particularly in late summer.
River Flow: While the flow of a river is in constant flux, it is from mid July to mid September where we see water flows play such a vital role in the productivity of a fishery. Too high and everything gets blown out leaving the fish scrambling trying to find suitable habitat. Too low and the fish begin to get stressed because they feel more vulnerable to predators. So it seems that we're always looking for that elusive "prime" flow where the bugs are hatching, and the fish are happily feeding. So how do we know when that "prime" flow is? Unfortunately there isn't a set standard or method to measure this; every river is different, and each will react differently to the rise and fall in flow. But if we understand where the water that fills our streams comes from, it becomes easier to calculate when these "prime" flows will be, particularly during the late summer months.
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