Freestone Trout Streams - Aquatic Insects
When the rain and snow falls from the sky it is pure and free from minerals and
is slightly acidic. It usually has a pH of approximately six. This can be affected by
acid rain. The intent here is to show that the pH changes from the headwaters to
the slower moving water in the foothills and how this affects the aquatic insect life.
As the water flows downhill, the pH will usually increase depending upon the
rocks, sand, gravel, and organic material (such as leaves and vegetation) the
water passes through. The pH of the headwater streams varies from region to
region depending on the composition of the soil and rocks.
Rain forest type terrain, such as is found in the Appalachian Mountains provides
the water a different chemical composition than the more barren slopes of the
Rocky Mountains, for example. If water passes through volcanic rock, it is going
to remain acidic much longer than water passing through a forest. The pH of the
water in the forest changes from the headwaters to the foothills.
The different pH values of the water from its origin in the mountains to the larger
streams or river in the valleys supports different groups of aquatic insects. The
water temperature is generally higher in the lower sections of the stream and this
can also be a factor that affects trout in that it changes the insect population.
The speed of the water is also a big factor in determining which aquatic insects
exist. Insects found in the fast flowing pocket water of the headwaters may be
quite different from those found in the slower moving water found at the lower
Because the water is usually fast moving pocket water, mayflies found in the
headwaters are usually clingers. Caddisflies are not very plentiful in these waters
because of the acidic level of the water and consequent low algae levels.
Many species of stoneflies are in their prime habitat in the highly oxygenated
water. This water, which is usually slightly acidic, will not support plant life such
as algae. The aquatic insects must rely on other source of food.
When the stream becomes the "run, pool, riffle" type of stream, normally found in
the foothills, the more diverse type of water will usually support other many other
species of mayflies, caddisflies and stoneflies. Mayflies may include several
species of crawlers and swimmers. The caddisfly population and diversity will
increase and include many species of scrapers, predators, and shedders due to
the diverse type of habitat. Stoneflies are still usually present in the fast water.
Since the water has poured through rocks, gravel, sand and other types of soil
and since organic material such as leaves may have accumulated in the stream,
the water is less acidic than it is in the headwaters. It will normally support specie
of aquatic insects that rely on organic material that has become more prevalent
due to the higher pH. Its increased temperature is also conductive to supporting
After the stream reaches the valleys it slows down and congregates in larger
pools with riffles. Since the water temperature is much warmer than the water at
higher elevations and since the pH has become even more alkaline, plant growth
may be present. The stream's substrate usually consists of more soil and less
rocks and burrowing mayflies may exist. The caddisfly population may increase
since there is a lot more organic material available for the larvae. Shedders,
predators and scraper species may be prevalent. Stoneflies my not exist in the
less oxygenated warmer water.
Copyright 2013 James Marsh
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