Geoscience Reference
In-Depth Information
such as the Grand Canyon of the Colorado river, or at
passive margins. Gorges tend to be straight or of
low sinuosity in orogens, closely directed by geological
structure. By contrast, uplift of the lower reaches of
established meandering rivers, or their rejuvenation by
falling base levels, often conserves sinuous channels as
incised meanders
. The Dee gorge has straight and incised
meander sections, with
abandoned meander cores
250-300 m deep, along a 20 km stretch between Corwen
and the English border east of Llangollen. Impressive
waterfalls represent either uplift rates exceeding incision,
in which case they form
knick points
marking the
upstream limit of adjustment (
Plate 14.14
),
or between
incision and geological structure. The Angel Falls,
plunging 1,000 m off the Roraima plateau of Venezuela,
exemplify the former and the latter include the 52 m high
Niagara Falls incising the Niagara escarpment.
In older, lower mountains and uplands, fluvial channels
dominate the narrow floors of valleys with V-shaped
cross-sections and rock slopes barely concealed beneath
thin regolith. Valley floors are usually irregular. Steep
sections, marked by waterfalls and associated plunge
pools, rapids, potholed bedrock channels and angular
boulders, separate gravel-lined stretches in profile
concavities. Tributaries enter higher-order rivers across
small, steep
debris cones
or more extensive and less steep
alluvial fans
, which mark significant reductions in channel
slope and stream competence. This applies on the grand
scale along mountain fronts, where they may coalesce
into huge
piedmont fans
10
2-4
bajadas (
Plate 14.15
).
Much of the later Cenozoic
molasse
south of the Himalayan mountain front was formed in the
former way.
Most temperate catchments reflect the geomorphic
impact of Pleistocene cold stages. Glacial excavation of
deep valleys and permafrost ornamentation of slopes in
upper catchments disrupted the fluvial landsystem (see
Chapters 15
and
25).
Glacial diversion of drainage and
sedimentation altered drainage networks in lowland
catchments. High spring glacial or permafrost meltwater
discharge eroded enlarged valleys. They survive as
dry
valleys
or are occupied now by diminutive
misfit streams
,
especially in normally porous lithologies such as the chalk
and limestone dominating the uplands of south-east
Britain. Altered catchment slope and sediment dynamics
still influence fluvial processes in most areas today (
Figure
Lakes represent a further, transient interruption of
fluvial development of the catchment. They occur where
downstream flow is impeded by rock or debris barriers
km
2
in area, or arid-zone
Mount Rakaposhi (7,788 m OD) into the Hunza valley,
Karakoram range, Pakistan, across a huge fan above the break
of slope. Continuing tectonic uplift and intense glacial and
fluvial erosion distinguish this place, together with Nepal, as
the steepest on Earth.
Photo: Andrew Goudie
