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How do rivers create valleys through erosion?

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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.

Rivers create valleys primarily through the process of erosion, acting as the primary agents that carve the landscapes through which they flow. As water moves, it scours the riverbed and entrains sediment, which then serves as a tool for further abrasion, grinding away rock and soil.123

During the initial stages of development, rivers expend most of their energy in vertical entrenchment, creating narrow, deep valleys. As the river continues to downcut, it eventually smooths its longitudinal profile and reaches an elevation closer to its baselevel. At this point, the river expends more energy laterally, which progressively broadens the valley floor.4

While most valleys are the result of river erosion, some are formed by tectonic activity, such as rift valleys, where the low topographic zone existed before the river. Additionally, glacial valleys are often distinguished by their characteristic U-shaped cross-sections, which differ from the typically V-shaped valleys carved by rivers.15