Science of Water : Concepts and Applications

Water Ecology 179
THE FLOODPLAIN
A stream channel infl uences the shape of the valley fl oor through which it courses. The self-formed,
self-adjusted fl at area near the stream is the fl ood plain, which loosely describes the valley fl oor
prone to periodic inundation during over-bank discharges. What is not commonly known is that
valley fl ooding is a regular and natural behavior of the stream.
A stream’s aquatic community has several unique characteristics. The aquatic community operates
under the same ecologic principles as terrestrial ecosystems, but the physical structure of the
community is more isolated and exhibits limiting factors that are very different than the limiting
factors of a terrestrial ecosystem. Certain materials and conditions are necessary for the growth
and reproduction of organisms. If, for instance, a farmer plants wheat in a fi eld containing too little
nitrogen, it will stop growing when it has used up the available nitrogen, even if the wheat’s
requirements for oxygen, water, potassium, and other nutrients are met. In this particular case, nitrogen
is said to be the limiting factor. A limiting factor is a condition or a substance (the resource
in shortest supply), which limits the presence and success of an organism or a group of organisms
in an area. Even the smallest mountain stream provides an astonishing number of different
places, or habitats, for aquatic organisms to live. If it is a rocky stream, every rock of the substrate
provides several different habitats. On the side facing upriver: organisms with special adaptations,
that are very good at clinging to rocks, do well here. On the side that faces downriver, a certain degree
of shelter is provided from the current but it still allows organisms to hunt for food. The top of a rock,
if it contacts air, is a good place for organisms that cannot breathe underwater and need to surface
now and then. Underneath the rock is a popular place for organisms that hide to prevent predation.
Normal stream life can be compared to that of a “balanced aquarium” (ASTM, 1969). That is, nature
continuously strives to provide clean, healthy, normal streams. This is accomplished by maintaining
the stream’s fl ora and fauna in a balanced state. Nature balances stream life by maintaining both the
number and the type of species present in any one part of the stream. Such balance ensures that there
is never an overabundance of one species compared to another. Nature structures the stream environment
so that both plant and animal life is dependent upon the existence of others within the stream.
As mentioned, lotic (washed) habitats are characterized by continuous running waters or current
fl ow. These running water bodies have typically three zones: riffl e, run, and pool. The riffl e zone
contains faster fl owing, well-oxygenated water, with coarse sediments. In the riffl e zone, the velocity
of the current is high enough to keep the bottom clear of silt and sludge, thus providing a fi rm bottom
for organisms. This zone contains specialized organisms, which are adapted to live in running
water. For example, organisms adapted to live in fast streams or rapids (trout) have streamlined
bodies that aid in their respiration and in obtaining food (Smith, 1974). Stream organisms that live
under rocks to avoid the strong current have fl at or streamlined bodies. Others have hooks or suckers
to cling or attach to a fi rm substrate to avoid the washing-away effect of the strong current.
The run zone (or intermediate zone) is the slow-moving, relatively shallow part of the stream
with moderately low velocities and little or no surface turbulence.
The pool zone of the stream is usually a deeper water region where velocity of water is reduced
and silt and other settling solids provide a soft bottom (more homogeneous sediments) that is
unfavorable for sensitive bottom-dwellers. Decomposition of some of these solids causes a lower
amount of DO. Some stream organisms spend some of their time in the rapids part of the stream
and others in the pool zone. For example, trout typically spend about the same amount of time in
the rapid zone pursuing food as they do in the pool zone pursuing shelter.
Organisms are sometimes classifi ed based on their mode of life.
1. Benthos (mud dwellers)—The term originates from the Greek word for bottom and
broadly includes aquatic organisms living on the bottom or on submerged vegetation.
They live under and on rocks and in the sediments. A shallow sandy bottom has sponges,
snails, earthworms, and some insects. A deep, muddy bottom will support clams, crayfi sh,