(a) Alluvial Fans. Where streams flow from step slopes onto bordering lowlands, the abrupt drop in
gradient causes an associated loss in velocity. This results in the deposition of an alluvial fan, which is a fan-
shaped feature with its apex located at the point where the stream emerges from the mountainous region. With
further deposition, adjacent fans may grow and overlap one another, forming a continuous piedmont alluvial
plain, such as the one shown in figure 2-7, page 2-14.
Alluvial fans are most commonly formed in arid or semiarid regions where streambeds are frequently dry except
after torrential cloudbursts. For example, fans of enormous dimensions have developed along the margins of
fault-block mountains in the western and southwestern United States.
(b) Channel Deposits. As previously mentioned, when water flows through a meander, erosion
takes place along the channel wall on the outside of the curve, while deposition occurs on the inside of the curve.
Therefore, the entire stream gradually migrates in the direction of the outer bank. As the stream migrates, the
deposits along the inside of the curve grow laterally, forming a teardrop-shaped deposit known as a point bar (see
figure 2-6, page 2-11). Point bar deposits are generally made up of relatively coarse-grained (sand- to gravel-
sized) particles. During flood stages, the stream may cut across the meander, leaving crescent-shaped abandoned
channels called oxbow lakes (see figure 2-6, page 2-11). Channel bars are similar to point bars in that they too
form as a result of a decrease in stream velocity; however, they occur in the interior of the channel rather than
along the edge, as in the case of a point bar. Channel bars normally contain relatively coarse-grained materials; it
is unlikely that these types of deposits will be found where the sediment source is composed of clay, shale, or
other fine-gained material. In instances where tributaries deliver a greater amount of sediment than the main
stream can transport, a large number of channel bars are deposited. In some cases, the entire channel consists of a
series of branching and reuniting streams flowing around and among the bars. These types of streams are known
as braided streams (see figure 2-8, page 2-15).
Due to the generally coarse-grained nature of channel deposits, construction materials are commonly excavated
from riverbeds. However, care should be taken in excavation because a borrow pit in the riverbed intercepts
transported material and, at the same time, increases downstream scour. For this reason, excavations in the
vicinity of structures, such as bridge piers, should be avoided. In general, it is best to locate borrow pits as far
downstream from important river installations as possible.
(c) Overbank Deposits. Swiftly moving streams and rivers are capable of carrying large amounts of
unconsolidated sedimentary material. However, when the stream level rises to the point where it overflows its
banks (as in a flood), the velocity decreases. This initially causes the deposition of relatively large, heavy
particles along the banks, followed by progressively smaller, lighter-weight particles at greater distances from the
channel. The two long, narrow, coarse-grained ridges bordering the stream channel are called natural levees.
Because of their composition, natural levees are often good sources of construction material, such as sand and
gravel. The finer-grained (silt and clay) deposits form floodplains. In terms of aerial extent, floodplains are the
primary depositional forms associated with fluvial systems. Figure 2-9, page 2-16, illustrates some common
depositional features of running water.
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EN5341
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