|....................... ......Fly Fishing Yellowstone National Park
Freestone Streams - Part 2
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. 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 of the East 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 different pH values of the water from its origin in the mountains too 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 may not be very plentiful in these
waters if the water comes of barren slopes above the timberline 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 streams reach the plateaus, there is much more plant life and a
different soil composition. The pH increases and more aquatic insects will be
found. 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.
Copyright 2009 James Marsh