.......................  ......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. The pH will change
from the headwaters to the slower moving water in the foothills. This will have a
big effect on 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 but all in all in the
volcanic rock of Yellowstone National Park, it doesn't vary all that much. Rain
forest type terrain, such as is found in the Appalachian Mountains, including the
Great Smoky Mountains National Park, provides the water a different chemical
composition than the more barren slopes of the Rocky Mountains. 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 flowing through the forest
changes from the headwaters to the foothills. Of course there are many forest
areas in Yellowstone and these affect the pH of the water differently than the
other type of areas.

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 elevations.

Continued tomorrow....

Copyright 2009 James Marsh