River Types: Introduction
Classifying a cold, flowing body of water according
to its geology, chemistry, and biological life forms that live
within it is often difficult, due in large part to the fact that
there are so many different terrains over which they flow. Designating
a moving body of water as a river, stream, or creek is based primarily
on size and volume. Rivers are larger than streams, which are
usually larger than creeks. Regardless of what we call them, each
takes on the chemical characteristics of the dissolved portion
of the ground (substrate) over which it flows, making each one
unique compared to all others. A river is a linear gradient
with respect to the amount of dissolved oxygen, pH
(acidic or basic), particulates, temperature, flow rate, and a
host of other physical and chemical characteristics, all of which
vary from its source to its mouth. These characteristics, in combination,
select for an incredibly varied series of niches.
The life forms that live there have tolerance
limits that fall within the local environment of each moving
body of water.
Each day brings with it a new set of external conditions to influence
those conditions. The
riparian-terrestrial environment also contributes to the outcome
of each moving body of water, confounding our ability to classify
them even further. However, some features that are shared by most
can be used to group cold, moving bodies of water into just a
few general types, particularly when they are keyed to the presence
or absence of salmonids
and their food
webs. The vast majority of rivers and streams begin as runoff
from snow melt and rain in areas of high elevation, while another
large sub-set begin their journeys to the sea in more gentle surroundings,
such as springs that bubble up out of the ground. Increasingly,
many are altered by damming up free flowing waters (tailwater
fisheries) and the tailwaters below them have produced some
of the most amazing trout habitat on earth.
