Fluvial Landforms
Stream Channel Types
Within a single stream we can often recognize three different channel types. These unique
channel types develop in response to changes in stream velocity, sediment texture, and stream
grade.
Channels located in the upper reaches of many streams tend to be narrow with flow moving at
high velocities (Figure 10z-1). The high flow velocities found in these streams are the result of a
steep grade and gravity. Within these stream systems, erosion is a very active process as the
channel tries to adjust itself to the topography of the landscape. Deposition occurs primarily
during periods of low flow. As a result, floodplain deposits are very limited, and the stream bed
is very transient and shallow.
Figure 10z-1: Upper reach of a stream in the Rocky Mountains, Canada.
Streams with high sediment loads that encounter a sudden reduction in flow velocity generally
have a braided channel type (Figure 10z-2). This type of stream channel often occurs further
down the stream profile where the grade changes from being steep to gently sloping. In a
braided stream, the main channel divides into a number of smaller, interlocking or braided
channels. Braided channels tend to be wide and shallow because bedload materials are often
coarse (sands and gravels) and non-cohesive.
Figure 10z-2: Braided stream channel.
Meandering channels form where streams are flowing over a relatively flat landscape with a
broad floodplain (Figure 10z-3). Technically, a stream is said to be meandering when the ratio
of actual channel length to the straight line distance between two points on the stream channel is
greater than 1.5. Channels in these streams are characteristically U-shaped and actively migrate
over the extensive floodplain.
Figure 10z-3: Meandering stream channel.
Stream Channel Features
Within the stream channel are a variety of sedimentary beds and structures. Many of these
features are dependent upon the complex interaction between stream velocity and sediment size.
Streams carrying coarse sediments develop sand and gravel bars. These types of bars seen often
in braided streams which are common in elevated areas (Figure 10z-4). Bars develop in
braided streams because of reductions in discharge. Two conditions often cause the reduction in
discharge: reduction in the gradient of the stream and/or the reduction of flow after a
precipitation event or spring melting of snow and ice.
Figure 10z-4: Braided stream channel with gravel bars.
Point bars develop where stream flow is locally reduced because of friction and reduced water
depth (Figure 10z-5). In a meandering stream, point bars tend to be common on the inside of a
channel bend.
Figure 10z-5: Meandering stream channel as seen from above.
In straight streams, bar-like deposits can form in response to the thalweg (red arrows Figure
10z-6) and helical flow. Figure 10z-6 below shows an overhead view of these deposits and
related features.
Figure 10z-6: Overhead view of the depositional features found in a
typical straight stream channel.
In this straight channel stream, bars form in the regions of the stream away from the thalweg.
Riffles, another type of coarse deposit, develop beneath the thalweg in locations where the faster
flow moves vertically up in the channel. Between the riffles are scoured pools where material is
excavated when the zone of maximum stream velocity approaches the stream's bed. The absolute
spacing of these features varies with the size of the channel. However, the relative distance
between one riffle and the next is on average five to seven times the width of the channel
(exaggerated in diagram). Both of these features can also occur in sinuous channels.
Dunes and ripples are the primary sedimentary features in streams whose channel is composed
mainly of sand and silt. Dunes are about 10 or more centimeters in height and are spaced a meter
or more apart. They are common in streams with higher velocities. Ripples are only a few
centimeters in height and spacing, and are found in slow moving streams with fine textured beds.
Both of these features move over time, migrating down stream. Material on the gently sloping
stoss-side of these features rolls and jumps up the slope under the influence of water flow.
Particles move up the slope until they reach the crest of the feature and then avalanche down the
steeper lee-side to collect at the base of the next dune or ripple. This process is then repeated
over and over again until the material reaches a location down stream where it is more
permanently deposited.
The Floodplain
Alongside stream channels are relatively flat areas known as floodplains (Figure 10z-7).
Floodplains develop when streams over-top their levees spreading discharge and suspended
sediments over the land surface during floods. Levees are ridges found along the sides of the
stream channel composed of sand or gravel. Levees are approximately one half to four times the
channel width in diameter. Upon retreat of the flood waters, stream velocities are reduced
causing the deposition of alluvium. Repeated flood cycles over time can result in the deposition
of many successive layers of alluvial material. Floodplain deposits can raise the elevation of the
stream bed. This process is called aggradation.
Figure 10z-7: The following Landsat 5 image taken in September 1992
shows a section of the Missouri River at Rocheport, Missouri. The
oblique perspective of this image is looking westward or upstream. This
image has been color enhanced and modified to show an exaggerated
topographic relief. Bare soil and plowed land appears red, vegetation
appears green, and water is dark blue. A flat river flood plain can be
seen in the center of the image. Because of the season, most of the
farmland located on the rich and fertile soils of the floodplain is plowed
and devoid of vegetation. (Source: NASA Scientific Visualization
Studio).
Floodplains can also contain sediments deposited from the lateral migration of the river channel.
This process is common in both braided and meandering channels. Braided channels produce
horizontal deposits of sand during times of reduced discharge. In meandering streams, channel
migration leads to the vertical deposition of point bar deposits. Both braided and meandering
channel deposits are more coarse than the materials laid down by flooding.
A number of other geomorphic features can be found on the floodplain. Intersecting the levees
are narrow gaps called crevasses. These features allow for the movement of water to the
floodplain and back during floods. Topographical depressions are found scattered about the
floodplain. Depressions contain the some of the finest deposits on the floodplain because of their
elevation. Oxbow lakes are the abandoned channels created when meanders are cut off from the
rest of the channel because of lateral stream erosion.
Alluvial Fans and Deltas
Streams flowing into standing water normally create a delta (Figure 10z-8 and 10z-9). A delta
is body of sediment that contains numerous horizontal and vertical layers. Deltas are created
when the sediment load carried by a stream is deposited because of a sudden reduction in stream
velocity. The surface of most deltas is marked by small shifting channels that carry water and
sediments away from the main river channel. These small channels also act to distribute the
stream's sediment load over the surface of the delta. Some deltas, like the Nile, have a triangular
shape. Streams, like the Mississippi, that have a high sediment content and empty into relatively
calm waters cause the formation of a birdfoot shaped delta.
Figure 10z-8: Nile Delta (Source: NASA).
Figure 10z-9: Mississippi Birdfoot Delta (Source: NASA).
Most deltas contain three different types of deposits: foreset, topset and bottomset beds.
Foreset beds make up the main body of deltas. They are deposited at the outer edge of the delta
at an angle of 5 to 25 degrees. Steeper angles develop in finer sediments. On top of the foreset
beds are the nearly horizontal topset beds. These beds are of varying grain sizes and are formed
from deposits of the small shifting channels found on the delta surface. In front and beneath the
foreset beds are the bottomset beds. These beds are composed of fine silt and clay. Bottom set
beds are formed when the finest material is carried out to sea by stream flow.
An alluvial fan is a large fan-shaped deposit of sediment on which a braided stream flows over
(10z-10). Alluvial fans develop when streams carrying a heavy load reduce their velocity as they
emerge from mountainous terrain to a nearly horizontal plain. The fan is created as braided
streams shift across the surface of this feature depositing sediment and adjusting their course.
The image below shows several alluvial fans that formed because of a sudden change in
elevation.
Figure 10z-10: Alluvial Fans - Brodeur Peninsula, Baffin Island,
Canada. (Source: Natural Resources Canada - Terrain Sciences
Division - Canadian Landscapes).