Geology Reference
In-Depth Information
()
a
Flood
basin
Point
bars
Low-water
channel
Natural
levee
Flood basin
and backswamp
Terrace
of older
alluvium
Buried
channel
Embanked
channel beds
Buried
sand splay
Sand
splay
Flood level
Older alluvium
Horizontal and current-
bedded channel deposits
Flood silts
()
b
Flood silts
Active
deposition
Active
erosion
Ephemeral
suspended beds
Bed-load
deposits
Figure 9.8
Sections through floodplains. (a) A convex floodplain. Point-bar deposits occur on inside meander bends and
rarely opposite developing levees. The vertical exaggeration is considerable. (b) A flat floodplain.
Source:
After Butzer (1976, 155, 159)
River terraces
formed by being cut in turn on each side of the valley
(Figure 9.9b).
The floor of a river valley is a precondition for river ter-
race formation. Two main types of river terrace exist that
correspond to two types of valley floor: bedrock terraces
and alluvial terraces.
A
terrace
is a roughly flat area that is limited by slop-
ing surfaces on the upslope and downslope sides. River
terraces are the remains of old valley floors that are left
sitting on valley sides after river downcutting. Resistant
beds in horizontally lying strata may produce flat areas
on valley sides -
structural benches
- so the recogni-
tion of terraces requires that structural controls have been
ruled out. River terraces slope downstream but not nec-
essarily at the same grade as the active floodplain.
Paired
terraces
form where the vertical downcutting by the river
is faster than the lateral migration of the river channel
(Figure 9.9a).
Unpaired terraces
form where the chan-
nel shifts laterally faster than it cuts down, so terraces are
Bedrock terraces
Bedrock
or
strath
terraces start in valleys where a river
cuts down through bedrock to produce a V-shaped val-
ley, the floor of which then widens by lateral erosion
(Figure 9.10). A thin layer of gravel often covers the
flat, laterally eroded surface. Renewed downcutting into
