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Stream
Ecology
by
Carl Prophet
PHYSICAL
CHARACTERISTICS OF STREAMS
The
ecology of a stream is influenced by numerous features.
A stream is an open system. That is, a stream is not isolated
from its immediate surroundings. It receives runoff from
its drainage which carries both suspended and dissolved
materials into the stream. The quality of the water is impacted
by the dominating geology of its drainage as well as the
surrounding land use. It may be said that whatever goes
on within a stream's drainage may ultimately affect the
ecology of that stream. Rather than existing as individual
isolated channels, streams within an area merge into a stream
system. If one were to view a river drainage basin from
above, it would become apparent that the many drainage channels
within the basin form a dendritic pattern in which
smaller streams join to form progressively larger streams
as one moves downstream.
During
the 1930s Robert E. Horton introduced a system for designating
the rank of individual tributaries within a river system.
For example, small streams that have no tributaries are
designated as first order streams. A second order stream
results when two first order streams join, and a third order
stream is formed by the joining of two second order streams,
and so on. A second order stream may actually have multiple
first order tributaries and so too may each of the higher
ranked streams have multiple lower ranked tributaries. In
general, the higher the order the greater the drainage basin
of a stream. The Cottonwood and Neosho rivers above their
confluence a few miles southeast of Emporia are fourth order
streams, and below the confluence the Neosho River becomes
a fifth order stream. The Mississippi River is classed as
a tenth order stream. This is the highest order reported
for any North American River.
Photo
caption: Sauble Spring in Chase County is one of the sources
of Cedar Creek.
Most
Kansas streams originate instate. Some streams originate
from the outflow from a large spring, and while springs
do contribute to the discharge of many streams, most of
their discharge is derived from surface runoff. Each stream
within a system will have an individual watershed from which
it receives runoff. When the soils within a watershed become
saturated the surplus water begins moving downslope and
is eventually collected in a stream channel. The water mass
flowing down a stream channel does not flow as a uniform
single current but rather as a turbulent flow. Depending
upon the composition of the substrate the flowing water
erodes the sides and bottom of the channel and constantly
changes the physical features of the channel. Depending
upon the dominating geological formations in an area, the
substrate of a stream can be either sand, silt, gravel,
or rock and cobble and combinations thereof. The type of
substrate in a stream will control, to a degree, the kinds
of bottom dwelling organisms that inhabit the stream. The
geology and land uses in an area will also affect the chemistry
of the stream.
Many
streams exhibit an alternating succession of riffles
and pools. Riffles are formed along reaches of a
stream channel that cut across rocky ledges that are relatively
resistant to erosion or reaches where there is a buildup
of rocks and gravel. Here the water depth is shallow and
the velocity higher than in deeper reaches of the stream.
Large rocks and cobble may be strewn throughout the riffle
area which causes the flow to be visibly turbulent. Pools
form below riffles in areas where the substrate is more
easily eroded and the gradient is perhaps less than that
along the riffle's reach. The water is deeper and velocity
less. Thus, riffles and pools represent two quite different
microcommunities. Some stream organisms are found
primarily in riffles and others are associated mainly with
pool habitats. The lengths of individual riffle and pool
reaches are highly variable, but their locations along a
stream tend to remain relative stable for long periods of
time. This pattern of riffle-pool-riffle is repeated throughout
the length of most lower order streams but may be less evident
or even disappear in higher order streams. The pattern may
also be less evident in sluggish sandy or silty streams,
but even here the deposition of sand and silt bars and snags
form barriers that may create pool and riffle areas.
Photo caption: This stream channel is gradually moving to
the left as the bank on the outer part of the meander is
cut away as sand is deposited along the inner curve.
Besides
altering the physical appearance of a stream, riffle have
other impacts on the ecology of a stream. The turbulent
flow of water through a riffle aerates the water increasing
the dissolved oxygen content of the water in unpolluted
streams to levels that are higher than levels usually found
in nearby ponds and lakes. The turbulence also increases
evaporation which has a cooling affect and mixes dissolved
chemicals throughout the water volume. Thus, not only are
streams usually well oxygenated but they tend to be cooler
and less prone to developing marked seasonal differences
in temperatures and dissolved chemicals between surface
and bottom than conditions often found in ponds and lakes.
Three
types of stream channels are recognized: straight,
braided, and meandering. Stream channels are
rarely straight for long distances. In general, hydrologists
consider a stream channel straight between two points if
the distance the stream flows does not exceed 1.2 times
the direct line distance. A straight channel will usually
occur along reaches where the stream banks consist of erosion-resistant
rock and/or where the gradient is extreme. A braided channel
develops in areas where the channel becomes shallow and
wide and the water velocity slows such that the sediment
load can no longer be carried. The sediment is deposited
in long narrow parallel bars that become cross-cut to form
a network of intertwined channels. This type of channel
is most likely to form in sandy or silty bottom streams.
Most streams will exhibit a meandering channel in which
it bends first in one direction and next in the opposite
direction. Where the channel bends the velocity of the water
flowing on the inside of the curve is lower compared to
the outside of the bend. Because the lower the velocity
the less suspended materials the water can carry, material
is deposited on the inner curve creating a silt/sand or
gravel bar. The higher velocity next to the outer bank undercuts
that side of the channel resulting in the bend becoming
more exaggerated. Eventually, the channel may loop back
on itself and cut a new connecting channel. If the ends
of the old meander become sealed by the deposition of silt
and accumulation of detritus the result is an oxbow lake.
Photo caption: Deepest part of channel (Thalweg) shifts
from side to side as evidenced by this series of sand/gravel
bars that were exposed along the Neosho River at Emporia
during low discharge.
Hydrologists
have learned that it is the nature of streams to form meandering
channels over time. The water flowing down a stream channel
does not flow as a linear, uniform current. Friction between
the channel substrate and the water moving over it causes
a sort of spiraling helical current. In fact the velocity
of the very thin layer of water in contact with the substrate,
the boundary layer, is virtually zero. Little or
no current along the bottom is extremely important to many
benthic organisms because they are not subjected to the
effects of turbulence. Rather than moving downstream in
a straight line the helical current moves in a serpentine
fashion. As one moves downstream the deepest part of the
channel, the thalweg, moves from side to side rather
than staying in the middle of the channel. This flow pattern
is often revealed along a relatively straight reach of a
stream during periods of low discharge by the appearance
of sand or gravel bars seemingly evenly spaced and alternating
from side to side of the channel. It is clear that a stream
is a dynamic system.
The
flow in most of the major streams in Kansas has been interrupted
by the construction of large dams and reservoirs. Most of
these structures were created for flood control purposes.
Gradual drawdown of the impounded water helps maintain downstream
flow during periods of low runoff. Even with these dams
in place the annual discharge in Kansas streams tends to
be highly variable. Runoff following storm events may result
in bank-full to flood conditions. Such storm discharges
greatly alter the stream community. The force of the storm
discharge can move even large rocks down the channel and
sweep away many of the stream organisms. However, the disrupted
populations soon recover. Surface flow may cease in some
small streams during periods of little or no precipitation
with only a chain of isolated pools remaining along the
channel. Although the stream bottom between pools may appear
to be dry there may be water a few centimeters below the
surface. Some organisms survive by burrowing down to the
saturated substrate. If there is no input from springs and
the period of drought long, small streams may become completely
dry and the organisms inhabiting them perish. Obviously,
organisms inhabiting a stream community face many challenges.
Photo caption: Chikaskia River is mostly a sand/silt bottom
stream in Sumner County.
Next
Section: biological
features of streams
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