Geoscience Reference
In-Depth Information
sediment derived by fluvial erosion of bedrock, debris
delivered to channels from adjacent slopes and reworking
of unconsolidated fluvial sediments. Bedrock channels are
proof of fluvial erosion and channel incision maintains
potential energy on valley slopes. Despite the great depth
and angular profile of many incised bedrock channels,
fluvial erosion is not fully understood. The Colorado
river, over 1ยท5 km deep in the Grand Canyon (Arizona),
is the most celebrated example (
Plate 14.10
),
but bedrock
segments found in most upland rivers demonstrate its
principal processes and effects.
Corrosion
, or the removal of soluble minerals, and the
abrasion
(=
corrasion
) of particles moving against bedrock
have limited impact beyond the smoothing of channel
walls. Corrosion depends on rock susceptibility, water
velocity and discharge, but most dissolved load is probably
acquired from pre-channel processes, since water spends
relatively little time in channels. Abrasion depends on bed
shear stress, flow turbulence and relative rock hardness.
Channel
potholes
containing smoothed pebbles exemplify
the general process, known as
evorsion
through its
dependence on a fluid vortex (
Plate 14.11
).
However, the
ability of large entrained boulders to strike off angular
bedrock fragments in turbulent, high-velocity, high-
discharge flows is probably more effective in maintaining
angular profiles.
Time
Time
Stream long profile
Time
individual flood hydrographs. PDV and lag time vary according
to shape, network and slope. Note how PDV falls as lag times
increase, conserving the 'area under the curve' or total
discharge.
Source: After Gregory and Walling (1973)
water. Most material, however, is delivered to the channel
by mass wasting on surrounding slopes. Sediment is also
derived from anthropogenic ground disturbance by
agriculture, quarrying, construction and land degrada-
tion. General processes of entrainment, transport and
deposition outlined in
Chapter 12
have specific forms in
fluvial catchments and channels, where sediment is
transported as either
dissolved
,
suspended
or
bed
load.
Plate 14.10
The dramatic incision of rockwalled channels
has cut across Upper Palaeozoic and older rocks, covering up
to 1.7 Ga of Earth history, as the Colorado plateau was
elevated during the Neogene period (past 20 Ma) - also
triggering basin-range faulting to the west.
Photo: Ken Addison
Erosion
Continental denudation occurs through net erosion
in upper catchment areas and the removal of debris,
eventually to the oceans. Distinctions are drawn between
