Geography Reference
In-Depth Information
The development of stream valleys running transverse to landforms is a curious and
enigmatic phenomenon, since this opposes the basic law of nature that water flows
downhill. Many different situations can bring about such features. In one possibility, the
lowest outlets of the stream exist across the ridges, so the water simply backs up until
it spills to low ground and erodes a channel. In another, a stream erodes headward until
it cuts across the apparent obstacle. Two other possibilities are called antecedence and
superimposition (Thornbury 1969).
An antecedent stream is one that existed before the mountains were formed; when
uplift took place, the stream kept pace in downcutting and maintained its channel
through the mountains (Fig. 5.21). Antecedent streams have created some spectacular
valleys. The Brahmaputra and Indus Rivers originate on the Tibetan Plateau and flow
across the highest mountains on Earth to reach the sea. In the United States, the
Columbia River cuts through the Cascades, a major barrier to the Pacific Ocean. Super-
imposed streams develop where the former topography has been buried by sediments,
lava, or glacial deposits. New streams form on this surface, establish themselves, and
continue to erode through the underlying topography. Later, when the overlying softer
material is stripped away, the streams appear to have cut transversely through the to-
pography; in reality, the streams had no choice, since their pathways were established
under different conditions. The Hudson and Susquehanna rivers are among the better-
known superimposed streams.
FIGURE 5.21 East-west asymmetric valley in a subarctic alpine environment (Ruby Range, Yukon Ter-
ritory). The gentle slope on the left is south-facing. This valley was recently glaciated; unconsol-
idated material has accumulated largely since the melting of ice. Solifluction is the primary pro-
cess operating on the south-facing slope and accounts for the displacement of the lakes to the
south side of the valley. The major processes operating on the bare and rocky north-facing slope
are frost wedging, nivation, rockfall, and mudflow. The upper surface of this region is relatively
gentle, owing to cryoplanation. (Photo by L. W. Price.)
When streams do cut across mountains, they create water gaps. These are valuable
to people, since they provide easy passage across mountain barriers. The Columbia
Gorge through the Cascade Mountains, for example, contains roads and railways. Many
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