Environmental Engineering Reference
In-Depth Information
Ancient terraces
FIGURE 7.19
Uplift starts new erosion cycle, leaving old flood-plain
limits as terrace formations.
Present
river
very coarse-grained material and the river may change its course to cut a new, shallower
channel. The old channel remains buried
(
Figure 7.20)
and leaves no surface expression
indicating its presence. Because such a channel is often narrow, locating it during explo-
ration is difficult. The irregular bedrock surface presents problems for the founding of
piers or other foundations, and the boulder and gravel fill in the buried channel permits
high seepage losses beneath dams.
Stream Shape, Channel Characteristics, and Deposition
General
Stream shape often reflects the geologic conditions in the area through which, it flows as
well as providing indications of the types of materials deposited in its channel and flood-
plain. Currents, even in straight, confined channels, exhibit meandering, or swinging from
side to side in the channel. If the channel sides are erodible by the current, the channel
extends itself from side to side, and is said to meander. Meander development depends on
the erodibility of the stream banks and water velocity.
Straight Channels
Straight channels develop along weakness zones in the rock, or are constructed.
Crooked Channels
Crooked channels are characteristic of rock or hard clay beds that resist meandering. In
rock masses, forms are controlled by weakness planes or intrusions of hard rock into softer
rock (
Table 6.1).
The crooked shape of Rock Creek, Washington DC, which eroded
Precambrian crystalline rocks having varying degrees of resistance, is shown in
Figure
7.21.
In such youthful, steep-sided valleys, the stream flows either on the bedrock surface,
or over deposited coarse-grained materials including boulders, cobbles, and gravels.
Braided Streams
In braided streams, channel widths can range up to several miles, and are characterized by
an interlocking network of channels, bars, shoals, and islands, found generally in mature
streams. The larger braided streams are found downstream of glaciers or in the lower tracts
Present
river
FIGURE 7.20
Ancient buried valley in rock gorge filled with cobbles and
boulders.
Buried channel
