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The inspiration for this article stemmed largely from text found in Paul Weamer’s “The Bug Book.” This is arguably one of the most informational book’s I’ve ever read in regards to fly fishing, specifically entomology. If you’d like to purchase it, click here.

Insect Development on Freestone Streams

As defined by Weamer, a freestone stream is “a body of flowing water that is primarily fed by rainfall runoff or snowmelt and the acquired flows of other creeks and rivers.”

Freestone streams classify the majority of our rivers and streams in the United States, which makes it all the more important that you understand the characteristics of them.

On any river or stream, know that the warmest water is typically the first place insects begin to hatch. As the season progresses and air temperatures rise, water temperatures will rise as well, typically starting in the widest points of a stream making their way upstream.

In most cases, the section of a stream nearest the headwaters sees very little aquatic insect life; as cold temperatures and acidic water from snowmelt and rainfall runoff make it hard for insects to live.

Another contributor to cold temperatures is elevation; high elevation = cold water.

Because of the effect elevation has on water temperature, in some cases hatches in high elevation streams may occur two months later than other streams only several miles away at much lower elevations.

Due to variable temperatures found on freestone fisheries, hatch progression differs widely from tailwaters.

The same insect found hatching in a tailwater will develop at a much different pace than if it were in a freestone fishery.

The turbulent nature of freestone fisheries provides a well-oxygenated habitat for many species of insects to thrive in. Specifically, species such as caddis, clinger mayfly nymphs, and stoneflies.

While hatches on freestone fisheries aren’t as intense as tailwaters, insect variety is certainly there. The multitude of habitats found in freestone streams such as riffles, rapids, and long slow-moving pools offer a wide array of inhabitable space for various insects to call home.

Insect Development on Tailwaters

While freestone streams are certainly more prevalent in the U.S., tailwater fisheries offer some of the most profound fishing in the world.

Weamer says, “there are two types of tailwaters:

those that release cool or cold water from the bottom of the dam and those that release warmer water from levels higher in the dam.”

A thermocline serves as a barrier that prevents the suns rays from reaching past a certain depth, reservoirs with no thermocline flow warm all the time, regardless of where the dam releases.

However, deep lakes with a thermocline release cold water year-round.

Because power generation and release schedules manipulate water temperatures, they are susceptible to completely unnatural changes in relation to the time of year.

This is fine for the fish, but can make for a really hard time when trying to predict hatches.

If power generation or water release is halted for an abnormal period of time the timing of insect emergence can rapidly accelerate. Likewise, if power generation or water release is plentiful for an extended period, emergence can be rapidly decelerated.

Similar to freestone streams, the further you go upstream toward the release point, the greater the changes.

The virtually year-round cold water temperatures nearest the release point make it hard on some species of insects to survive, but the few that thrive in these conditions often hatch plentifully at times.