Geoscience Reference
In-Depth Information
Figure 16.30 The Nile River Delta.
This true-
color image shows the triangular shape of the Nile
River Delta where the Nile meets the Mediterranean
Sea. The color green represents dense foliage and
crops that are being supplied with water and nutri-
ents from the fertile soils of the Nile River Valley and
Delta. Note the surrounding desert.
system. With higher discharge the potential of flooding in-
creases, which will result in deposition of alluvium on the
active floodplain because the valley is wider than it is in the
higher reaches. Deposition of alluvium means that stream
terraces may also occur in this part of the profile, especially
if some waves of downcutting progressed through the area
in response to a drop in base level. The size of the sediment
load is usually lower in this part of the system compared to
the higher reaches due to a loss in competence.
Moving into the lowest reaches of the profile, the pat-
tern seen in the middle part of the system becomes even
more pronounced. Here, the trunk stream is fully devel-
oped, which naturally has a relatively high order because
it contains virtually all the water that lower-order tributar-
ies can provide. Stream gradient is very low here, resulting
in low stream power and the formation of well developed
meanders and a correspondingly wide valley. Discharge is
high, however, resulting in progressively greater width and
depth. Given that all the runoff in the drainage basin now
flows through this central conduit, the potential exists for
major flooding in this part of the system. Consequently,
the alluvium is probably very thick, and well developed
terraces are likely present. These landforms may be very
prominent if significant base-level fluctuations have oc-
curred through time.
lake or ocean. Such a place is often called the
river mouth
and
is a zone of intense sediment dispersal and deposition. This
shift in stream behavior takes places because stream velocity
slows dramatically when it enters the relatively still waters of
the larger water body. Think of what happens to your own ve-
locity when you run into a lake at the beach. It slows and you
then fall. When a stream slows in a similar setting, it drops its
sediment.
The landform created by sediment deposition at a river
mouth is called a
delta
because it usually has a triangular
shape similar to the Greek letter delta (Δ). An example of
this shape is the Nile River Delta (Figure 16.30). A delta
consists of a level plain of sediment that is sorted geographi-
cally by size. In other words, the coarsest sediments, such
as sands, are deposited at the point where the stream begins
to lose energy. As stream energy further weakens away from
the river mouth, the silt-sized fraction of the sediment load
is then deposited. The clay fraction is carried the farthest
in suspension and ultimately accumulates far from the river
mouth.
Another excellent example of a river delta is the
Mississippi River Delta in Louisiana, where the Mississippi
River meets the Gulf of Mexico. This delta is a classic
birdfoot
delta
because it appears to contain a number of individual toes
that can easily be seen in the satellite image (Figure 16.31a).
Each one of these “toes” is a river channel that is part of a
Deltas
The graded stream model considers the changes in
stream variables and behavior over the course of an idealized
longitudunal profile. Now let's examine what happens geo-
morphically at the location where some streams terminate. In
most cases, streams are tributaries of larger streams and sim-
ply flow into them at a confluence such as shown in Figure
A low, level plain that develops where a stream flows
into a relatively still body of water so that its velocity decreases
Delta
