Page 257 - Fundamentals of Geomorphology

240 PROCESS AND FORM

123,000 Terrace 275
frost
ossil
Fossil frost wedge years old 8
F
wedge
casts
and
casts and involutions
involutions
91,000 Terrace
years old
7
40,000,
80,000–90,000
years old
18,000–23,000 (m)
years old
250
Terrace Elevation
7,000 6
years old
3,200 Terrace
500–700 years old 5
years old
Terrace 4
Terrace 2 Terrace 3 Pre-Quaternary
Deposits
Modern
225
Figure 9.11 Terraces on the upper Loire River, France (diagrammatic).
Source: Adapted from Colls et al. (2001)
in the Swabian–Bavarian Alpine foreland, are climatically by confluence spurs on the upstream side of tributary
controlled terraces produced as the climate swung from valleys.
glacial to interglacial states and back again. The rivers
deposited large tracts of gravel during glacial stages, and
then cut into them during interglacial stages. Semi-arid Lacustrine deltas
regions are very susceptible to climatic changes because Lacustrine or lake deltas are accumulations of alluvium
moderate changes in annual precipitation may produce laid down where rivers flow into lakes. In moving from
material changes in vegetation cover and thus a big a river to a lake, water movement slows and with it
change in the sediment supply to streams. In the south- the water’s capacity and competence to carry sediment.
west USA, arroyos (ephemeral stream channels) show Providing sediment is deposited faster than it is eroded,
phases of aggradation and entrenchment over the last few a lacustrine delta will form.
hundred years, with the most recent phase of entrench-
ment and terrace formation lasting from the 1860s to
about 1915.
Terraces tend to survive in parts of a valley that escape HUMAN IMPACTS ON THE
erosion. The slip-off slopes of meanders are such a place. FLUVIAL SYSTEM
The stream is directed away from the slip-slope while
it cuts down and is not undercut by the stream. Spurs Human agricultural, mining, and urban activities have
at the confluence of tributary valleys also tend to avoid caused changes in rivers. This section will consider three
being eroded. Some of the medieval castles of the middle topics: the increased flux of fluvial sediments; the effect
Rhine, Germany – the castles of Gutenfels and Maus, for of dams on streamflow, sediment transfer, and channels;
example – stand on small rock-floored terraces protected and river modification and management.