Geoscience Reference
In-Depth Information
LONGITUDE
60°
120°
180°
120°
60°
0°
80°
80°
60°
60°
40°
40°
20°
20°
0°
0°
20°
20°
40°
40°
Less than 20mm
20 - 400
400 - 1000
over 1000
60°
60°
60°
120°
180°
60°
0°
120°
Source: After L'vovich (1973)
youthful headwaters and sluggish, senile lowland rivers.
However, both elements are present in mature, integrated
catchments. The profile corresponds to a power curve
requiring constant adjustment by the river to downstream
changes in potential energy, discharge, slope and sediment
load. Locally, surplus energy erodes and lowers the profile
through
incision
, whereas energy deficit leads to its
aggradation
or elevation through deposition. At any time
the profile is complicated by different stages of response
to tectonic and eustatic changes in base level, climatic,
geological and land-use conditions and - increasingly -
human regulation of the catchment.
Plate 14.13
Cross-bedded coarse sand, truncated at a
Photo: Ken Addison
Upper catchments and bedrock channels
Bedrock channels and associated valleys are not confined
to mountains or uplands, although tectonic drive and
denudation produce some of Earth's deepest gorges and
rock-walled valleys there. They are most dramatic in
alpine orogens, with gorges over 10
3
m deep in the upper
Tsangbo (Brahmaputra) and Indus trunk rivers and
principal tributaries such as the Hunza and Gilgit
(Karakoram range see
Plate 10.4
).
Other locations include
the middle reaches of rivers cutting through land surfaces
elevated by recent epeirogenesis in continental interiors,
lowland is seen by following the trunk river from source
to sea. Its typically concave long profile, along which
potential energy increases exponentially towards the
source, was once used to distinguish between vigorous,
