Environmental Engineering Reference
In-Depth Information
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 (Plate 14.11) and Gilgit (Karakoram range). Other locations include
the middle reaches of rivers cutting through land surfaces elevated by recent
epeirogenesis in continental interiors, 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
Plate 14.10
Cross-bedded coarse sand, truncated by fine and
medium gravel in fluvial sediments.
Photo: Ken Addison.
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, or between incision and geological structure. The Angel
Falls, plunging 1000 m off the Roraima plateau of Venezuela, exemplify the former and
the latter include the 52 m high Niagara Falls incising the Niagara escarpment.
