.......................  ......Fly Fishing Yellowstone National Park


Freestone Streams - Part 3

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
species 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 other insects.

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.

Continued tomorrow....

Copyright 2009 James Marsh