Page 253 - Fundamentals of Geomorphology

236 PROCESS AND FORM

() Flood Point Low-water Natural Flood basin Terrace
a
basin bars channel levee and backswamp of older
alluvium
Buried Embanked Buried Sand
channel channel beds sand splay splay
Flood level

Older alluvium Horizontal and current- Flood silts
bedded channel deposits
()
b
Flood silts
Active Active
deposition erosion

Ephemeral Bed-load
suspended beds 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).
A terrace is a roughly flat area that is limited by slop- The floor of a river valley is a precondition for river ter-
ing surfaces on the upslope and downslope sides. River
terraces are the remains of old valley floors that are left race formation. Two main types of river terrace exist that
correspond to two types of valley floor: bedrock terraces
sitting on valley sides after river downcutting. Resistant and alluvial terraces.
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 Bedrock terraces
ruled out. River terraces slope downstream but not nec-
essarily at the same grade as the active floodplain. Paired Bedrock or strath terraces start in valleys where a river
terraces form where the vertical downcutting by the river cuts down through bedrock to produce a V-shaped val-
is faster than the lateral migration of the river channel ley, the floor of which then widens by lateral erosion
(Figure 9.9a). Unpaired terraces form where the chan- (Figure 9.10). A thin layer of gravel often covers the
nel shifts laterally faster than it cuts down, so terraces are flat, laterally eroded surface. Renewed downcutting into