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How can stream capture cause a small river to occupy a valley that seems too large for it?

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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.

A river valley is typically formed by the erosive power of the river that flows through it, meaning the size of the valley generally corresponds to the size of the river. However, a phenomenon known as stream capture—or stream piracy—can disrupt this relationship.1

Stream capture occurs when a more powerful, competing stream erodes its drainage divide and diverts the headwaters of an adjoining, weaker stream into its own system. When a large river is diverted away from its original path through this process, the valley it once carved remains behind. The original, expansive valley is then left to be occupied by a much smaller stream that lacks the discharge and erosive capacity to have created such a large feature. This resulting river is described as an underfit stream, as its volume is significantly reduced compared to the scale of the valley it occupies.123