Geoscience Reference
In-Depth Information
Cirque
moraine
Late Devensian
scree
Precipitation
Airborne
debris
Evaporation
Suspended
sediment
plume
Ombrogenous
peat
Solifluction
terrace
Kame
terrace
Topogenous
peat
Drift-plugged
channel
Valley
peat
Buried
channel
Drumlins
Terrace
Outlet
channel
erosion
Marsh
expansion and
biogenic infill
Figure 14.26 The geomorphic impact of Pleistocene cold
stages on upland catchments. Modern rivers rework a
landform and sediment system largely not of their making.
Source: Newson (1981)
Delta
Material fluxes
Laminated
clays and silts
(including varves)
Sediment
Water
Figure 14.27 Morphology, material fluxes and development
of a lake system. Relative inputs-outputs of the water and
sediment fluxes (including dissolved load) are shown by the
size of the arrows. More sediment enters than leaves the
system, leading to its eventual infill, and outlet lowering
progressively lowers the water level.
and are not exclusive to upper catchment areas. Confining
valley slopes create favourable sites for impounding
water after glacial excavation, landslide activity and
volcano-tectonic uplift. The vast lowland lake systems of
north-west Canada and eastern Scandinavia, and the
Great Lakes of North America, are largely glacial in origin.
Lakes are both fed and drained by rivers but they buffer
downstream stretches from sediment influx, which
progressively infills the basin instead, and also form
temporary base levels for upstream reaches ( Figure 14.27 ).
to maintain a more direct line in the 'senile' stage of the
river. Straight channels are uncommon, except in heavily
regulated and 'channelized' rivers where flood evacuation
is a required aim. Ironically, this speeds water on to the
next downstream unprotected zone, and new trends in
river management include meander restoration (see box,
p. 333). Straight channel segments carry low bed loads,
compared with meandering forms. Many natural channel
segments may appear straight but the thalweg , the line of
maximum water or channel depth, itself meanders. Flow
or channel sinuosity is measured as the ratio between the
channel and straight-line distances between two points
and = 1 in straight channels, increasing with sinuosity.
Meandering is arbitrarily considered to occur where
sinuosity is greater than 1·5.
Many theories have been advanced for meandering,
including chance deflection by obstacles leading to
destabilization of stream flow with repeated downstream
overcorrection. Recent studies emphasize temporal
changes in sediment supply and bed shear stress, stimu-
lating local erosion/deposition and hence new channel
geometry. Since channel efficiency increases downstream
to counteract lower potential energy, and velocity remains
constant, a downstream increase in discharge must require
adjustments to the channel. Meandering consumes
surplus energy by lateral erosion and friction with the
larger wetted perimeter implicit in sinuous channels
( Figure 14.28 and Plate 14.16 ).
Lower catchments and alluvial channels
Higher potential energy, leading to net denudation in
upper catchment areas, transfers large sediment volumes
into the lower catchment, where they line channels and
entire valley floors with alluvium . This is mostly the
sand-gravel fraction (0·06-60 mm) and above in straight
channels but includes substantial silt (0·002-0·006 mm)
and even clay ( 0·002 mm) fractions in sinuous
channels. Not all this material reaches its final marine
destination; the geological record reveals substantial
components of lithified terrestrial sediment in cratons or
incorporated in continental collisions. Alluvium is
reassimilated into New Zealand orogens before reaching
the Pacific Ocean (see Chapter 25). For all that, unlithified
alluvial sediments are far more easily eroded than bedrock
and facilitate channel adjustment to flow regime. The
flood plain environment also experiences dynamic changes
at whole-channel and floodplain scales, depositing and
remobilizing soft sediments.
Straight and meandering channels
Meandering is the natural tendency for alluvial channels,
although it was once thought to reflect sluggish inability
 
 
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