Geoscience Reference
In-Depth Information
mountain valleys and spread out across the piedmont or foot-slopes, rapidly losing
power and depositing poorly sorted coarse debris onto a growing alluvial fan.
Where the plant cover in the headwaters is dense and the rainfall regime is less
erratic, as in the seasonally wet tropics and many semi-arid areas, rock weathering
will be enhanced and deep soil mantles will cover many of the hill slopes and valley
sides, contributing fine sediments to the river channels from slopewash and soil creep.
The recipient stream channels will be suspension-load channels with deep and narrow
channel cross sections, cohesive beds and banks, and a meandering stream pattern.
Small ephemeral stream channels can change frombraided tomeandering within a few
years or decades in response to local hydrologic changes triggered by changes in plant
cover. Larger river channels can change from one type of channel to another during
longer intervals of time, depending on changes in stream discharge and sediment
load, a process described by Schumm (
1969
) as 'river metamorphosis'. We saw in
Section 10.4
that both stream power and sediment transport rate are proportional to
stream velocity cubed. Once stream power falls below a limiting threshold value, bank
erosion and sediment transport will diminish, leading to a change in channel pattern
from braided to meandering.
10.6 Quaternary paleochannels in semi-arid south-east Australia
The largest rivers in Australia today rise in the Eastern Highlands and flow west or
south-west across semi-arid alluvial plains. These rivers depend primarily on summer
rainfall in the north and spring snow-melt in the south. A series of former river
channels are clearly evident across the semi-arid Riverine Plain in south-east Australia
(
Figure 10. 9
). In a classic study of the Murrumbidgee River and its associated former
channels, Schumm (
1968
) examined borehole data and channel dimensions and sought
to explain how the changes in stream pattern and channel size reflected what he termed
'river adjustment to altered hydrologic regimen'.
Schumm observed that there were two distinct types of channel with quite different
types of sediments within them. He adopted the names
ancestral stream
and
prior
stream
that had been used by earlier workers for these two channel types. The ancestral
stream channels were sinuous with meander wavelengths several times those of the
present meandering channel, and were filled with mainly fine sediment, consistent
with their sinuous channel pattern. In contrast, the prior steam channels contained a
coarser channel fill and were linear in plan, with wide, relatively straight channels.
Schummconcluded that the sinuous ancestral channels were suspension-load channels
formed when the climate was wetter and bankfull discharge was several times greater
than they are today. He proposed that the prior stream channels developed under a
more seasonal flow regime, with sporadic episodes of very high discharge coming
from more sparsely vegetated headwaters.
