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How do rivers carve narrow and deep valleys during early valley development?
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The rugged Atlas Mountains surround a valley in Morocco.
During the initial phase of valley development, rivers carve narrow and deep valleys primarily by expending most of their energy in vertical entrenchment. This process is particularly pronounced in areas located well above the regional baselevel, where the river's erosive power is focused on cutting downward into the landscape.12
Several factors influence the formation of these narrow, deep forms:
- Geologic Resistance: Valleys tend to be narrower and deeper when cut into rocks that resist erosion, such as most igneous rocks, well-indurated sedimentary rocks (e.g., quartzites), and silica-rich metamorphic rocks.1
- Tectonic Activity: Recent tectonic uplift, or rejuvenation, can significantly accelerate entrenchment, leading to the creation of spectacular canyons and gorges.3
- Initial Erosional Features: As a river begins to incise, it develops inner channels through the coalescence of scour features. These inner channels, which are often narrow and gorgelike, eventually contain the entire river flow.4
As downcutting continues, rivers gradually smooth out the longitudinal profile of the valley floor, eventually transitioning from vertical incision to lateral erosion, which broadens the valley over time.1