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How does permeable rock reduce the formation of river valleys?

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Snow-capped, rugged Atlas Mountains encircle a green valley with scattered trees, terraced fields, and a few small buildings in Morocco. — valley The rugged Atlas Mountains surround a valley in Morocco.

Permeable rock, particularly soluble carbonate rocks like limestone, can reduce the formation and deepening of river valleys by diverting surface water into underground drainage systems.

As a stream cuts into these permeable beds, water begins to leak through joints and fractures, which gradually enlarge into cave systems. Over time, these subsurface passages expand until they are capable of carrying a larger portion of the stream's flow. Eventually, the cave system may capture the entire flow of the surface stream, especially during low-flow periods.

When the surface channel is deprived of its water, the process of vertical erosion ceases, and the feature becomes a dry valley. Because the river no longer flows through it to perform the work of incision, the valley stops deepening, its banks may collapse, and the channel becomes overgrown with vegetation.1