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Why do valleys usually become wider as rivers approach their base level?

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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.

Valleys typically widen as rivers approach their base level because of a fundamental shift in how the river expends its energy.

During the initial stages of development, when a river is far above its base level, it primarily uses its energy for vertical entrenchment, resulting in narrow, deep valleys. However, as the river continues to downcut, it gradually smooths out its longitudinal profile. Once the river reaches an elevation close to its base level, it transitions from vertical incision to lateral erosion.1

In this state, the river expends more energy moving side-to-side, which causes it to progressively broaden the valley floor. This lateral migration is often facilitated by the river's tendency to meander, where the river erodes the outer banks of its bends while depositing sediment on the inside, effectively carving out a wider path over time. Consequently, most river valleys evolve from narrow, steep-sided forms into broader ones as they reach maturity near their base level.123