LOTIC ECOSYSTEMS
Rivers and streams rarely display the vertical stratification patterns found in standing bodies of
water because of their turbulent flow. Although slight differences in temperature can exist between the
surface and bottom waters of deep lotic systems, the greatest changes take place as water moves
downstream. Flowing water systems frequently possess greater habitat heterogeneity than lentic
systems. They also are more permanent ecosystems on a geological or evolutionary scale. Both
heterogeneity and permanence tend to increase species diversity.
The area drained by a stream and all of its tributaries is called its watershed. Any rain that falls
within the watershed will pass through the main stream channel. The streams occupying a watershed
form a hierarchical network of channels that hold increasingly larger volumes of water as you move
toward the mouth. Ridges and hilltops act as divides that separate the watershed into individual
drainages. Watersheds are therefore composed of many smaller drainage basins.
Rowlett Creek, for example, is found in the
watershed of the East Fork of the Trinity River. Its
headwaters begin four miles west of McKinney in
west central Collin County at an elevation of 750ft.
The stream and its tributaries flow southeast for
twenty-six miles. It is joined by Cottonwood Creek
near 14th street between Plano and Murphy. Further
south, near Garland, it is joined by Spring Creek.
Until the late 1960s Rowlett Creek flowed into the
East Fork of the Trinity River in southwestern
Rockwall County. In 1970 it was diverted to empty
into Lake Ray Hubbard. The perennial stream is
intermittent in its upper reaches. The watershed
area of 137.6 square miles includes the cities of
McKinney, Plano, and Allen in Collin County;
Richardson and Garland in Dallas County; and
Rowlett in Rockwall County. The moderately steep
to gently rolling terrain is surfaced by black land
clay over the Austin Chalk formation.
Watershed of the East Fork of the Trinity River
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Because the southern portion of the creek was
subject to seasonal flooding, several dams were
built on the creek in the late 1960s and early
1970s. During this same period the dramatic
growth of Plano, Allen, Richardson, and Garland
changed the profile of the Rowlett Creek
watershed from primarily rural to highly urban.
Associated with this urbanization was increased
runoff and channel erosion.
Size Classification
When conducting a stream study, it is
Mean Annual discharge (ft3/sec) of Rowlett Creek from
1970-1998.
useful to describe the stream so that readers can get a mental picture of what you’re describing. Stream
classification helps to identify similarities and differences among streams. Stream order is a
classification of streams based on tributary junctions and has proven to be a useful indicator of stream
size, discharge, and drainage area. A stream's order is its rank, or relative position, within the watershed
network. On a topographic map showing all intermittent and perennial streams in a basin, the smallest
unbranched tributaries are designated order 1. Where two first-order streams join, a second-order stream
segment is formed; where two second-order segments join, a third-order segment is formed, and so on.
As stream order increases, other characteristics change, such as channel shape, drainage area, habitat,
and biological communities.
One difficulty with this classification scheme is in
deciding what constitutes a first-order stream, since tributaries
may be too small to be seen. Another problem is that it is
designed for a dendritic drainage system. In linear, elongated
systems, a stream may remain low order while growing
atypically large. For example, Cottonwood Creek parallels
Rowlett Creek, but has few major tributaries and never becomes
higher than 2nd order. Rowlett Creek by comparison has a
number of major tributaries and at its confluence with
Cottonwood Creek is a 4th order stream.
Classification of a drainage system using
stream order.
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An alternative method characterizes streams by
magnitude. As with stream order, two 1st magnitude
streams join to form a 2nd magnitude stream. However,
magnitude increases by one for each 1st magnitude stream
entering. At each confluence the resulting magnitude will
always be the sum of the magnitudes of the conjoining
tributaries.
Magnitude has the same problem as order in
defining a first magnitude stream. It is better at classifying
elongated systems and more accurately describes small
streams. Magnitude does become cumbersome for larger
th
streams. A 10 order river like the Mississippi River
Classification of a drainage system using
stream magnitude.
could have a magnitude of over 200.
Channel Types
There are three basic types of channels,
straight, meandering and braided.
Describing a channel by one of the
aforementioned terms does not mean
that the entire channel is straight or
otherwise. It simply means that some
portion of the channel can be described
in such a way. In fact, portions of a
stream may be straight, some
meandering and others
braided. Describing a channel as a
straight channel seems pretty obvious,
though rarely is a channel perfectly straight in nature. A meandering channel is one that takes twists
and turns over its length. The sinuosity ratio is used to determine whether a channel is straight or
meandering. The sinuosity ratio is the distance between two points on the stream measured along the
channel divided by the straight line distance between the two points. If the sinuosity ratio is 1.5 or
greater the channel is considered to be a meandering one. A braided channel is created when a stream
channel is divided into several smaller ones by the accumulation of in-channel deposits. This occurs
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when the load of flat stream channel is too great for the velocity or discharge. Or, seasonal fluctuations
in discharge expose in-channel deposits. Sand or gravel bars accumulate subdividing the flow of water
into many smaller channels. Braided streams are common in glaciated areas where melt water streams
choked with sediment is discharged at the snout of the glacier.
Riffles and Pools
A meander is a bend in a stream.
Stream channels meander through the
watershed in response to changes in
topography. At bends in the stream channel
centrifugal force moves water toward the
outside bank in a helical or spiral pattern.
During times of heavy runoff this undercuts the bank (cut bank) causing erosion. This tumbling water
scours out the bottom forming a pool. During times of low flow pools are sites of deposition. Sediments
in pools tend to be finer and more homogeneous. Sediment eroded from this area is deposited on the
opposite bank forming a point bar. Bars usually make poor habitats for bottom dwelling organisms
because of their unstable, shifting nature.
Deposition downstream of the pool causes
the streambed to rise. This portion of the
channel where relatively shallow, rapidly
flowing water occurs is a riffle. Sediments in
riffle areas tend to be more coarse and
heterogeneous.
Neck & Cutoff
A neck is the upland between opposing meanders of a stream. A cutoff occurs when the neck between
river meanders is eroded away and the meanders join to shorten the length of the channel. The slope of
the channel increases as well when the river shortens its length.
Oxbow lake & meander scar
A river cut-off results in a portion of the river isolated from the new channel called an oxbow lake.
Oxbow lakes are typically crescent shaped - like that of an oxbow. Groundwater seeping into the oxbow
maintains the lake. Some oxbows will drain or silt up due to deposition during floods. The remnants of
the oxbow are identified as a meander scar. Wetland and marshes are often found in the scar.
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Floodplain
A floodplain is the relatively flat area that borders a stream which is periodically inundated with water
during high flow periods. When excess runoff causes the stream discharge to increase beyond the
capacity of the channel, water spills out onto the floodplain. Increasing the cross-sectional area of stream
flow causes a decrease in stream velocity. The resulting decrease in velocity causes sediment to deposit
as alluvium on the floodplain. These alluvial deposits are often rich in nutrients and thus naturally
fertilize floodplain soils. Floodplain agriculture has given rise to many of the great world civilizations.
Natural Levee
A natural levee is a narrow ridge of alluvium deposited at the side of the channel. During high
discharge periods when the stream floods, coarse sediment settles out near the stream channel and
grades to finer material further away. The over bank deposits of alluvium are often rich sources of
nutrients for soils developed on the floodplain. Because floodplain soils are usually quite fertile, humans
have inhabited them for years. To prevent flooding, artificial levees are built close to the channel,
typically higher than natural levees. Confining the flood discharge to a small area increases the velocity
of flow. The levees of the Mississippi River increase the flow velocity near the mouth as it enters the
Gulf of Mexico.
Back swamp
Back swamps are located some distance away from the stream channel on the floodplain. When water
spills over onto the floodplain, the heaviest material drops out first and finest material is carried a greater
distance. The fine grained alluvium holds much water and drains rather slowly creating wetland areas.
Back swamps are important "sponges" that retain water that might cause severe flooding downstream.
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Stream Gradient
The longitudinal profile is a depiction of the down slope gradient of a stream. The longitudinal profile
of a stream can reveal whether a stream has achieved a graded state, whether over only a part or the
entire stream. The curved profile of a graded stream exhibits a steeper slope upstream giving way to a
gentle slope in the down valley direction. Initially stream profiles may be irregular with the stream
gradient interrupted by knickpoints where waterfalls are found. Knickpoints form where the stream
flows over an exposure of resistant bedrock or from tectonic uplift. The knickpoints slowly wear down
and migrate upstream as water spills over them. Through time the profile is smoothed to a gentle
concave shape.
Longitudinal Stream Profile
Flow velocity
The flow velocity of a stream is how fast the water is moving through a cross-section. Flow velocity is
determined by the balance between the down slope gravitational stress as a result of the slope of the
stream, and the loss or expenditure of energy in overcoming the frictional resistance of the channel bed
and side. In general, the flow velocity is greatest at the center of the channel, just below the surface.
More specifically the highest velocity of flow follows the stream thalweg, a line that connects the
deepest part of the stream channel. Here, water moving through the stream encounters the least
resistance to flow yielding a higher velocity of flow.
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The River Continuum Concept
Physical, chemical, and biological characteristics of a river change from headwaters to its mouth.
The RCC is a holistic view of river morphology, biotic assemblages, and ecosystem parameters that
describes consistent, predictable changes in freshwater habitats and trophic organization along a streams
course. It emphasizes that the lotic ecosystem includes the entire watershed- the channel itself, the
riparian zone, and the upslope drainages. Viewing the river as a continuum, it predicts that downstream
biotic communities are tightly coupled with upstream processes, such as detrital processing, FPOM
transport, and upstream disturbances. The RCC was developed in the eastern U. S. and is a typical of
forested eastern rivers and may not reflect conditions elsewhere, including the western U.S., where
channel geomorphology and biotic assemblages might differ considerably.
Woodland streams normally have headwaters in which the basin is narrow and heavily shaded,
with the water cool and shallow. Productivity is strongly influenced by riparian vegetation which
reduces autotrophic production, and contributes large amounts of allochthonous detritus (P<R). The
biota of headwaters is dominated by shredders and collectors which utilize the coarse particulate organic
matter present.
As stream order increases, the importance of terrestrial organic input is reduced, and a shift from
heterotrophic to autotrophic production takes place as shading decreases (P>R). Algae increase along
with fine particulate organic matter, which changes the trophic structure to grazers and collectors.
In large rivers the effects of riparian vegetation are insignificant, but primary production is often limited
by depth and turbidity. As a result, the increase in fine particulate organic matter from upstream
processing of dead leaves and woody debris shifts the stream back to a heterotrophic state (P<R).
Collectors dominate the biota.
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