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Why do waterfalls often form below hanging valleys?

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esker: Nunavut — glacial landform Eskers, or narrow ridges of gravel and sand left by a retreating glacier, wind through western Nunavut, Canada, near the Thelon River.

Waterfalls often form below hanging valleys due to the process of glacial erosion. During glaciation, a large trunk glacier has a greater capacity to erode the landscape than the smaller tributary glaciers that feed into it. Consequently, the main valley is deepened and widened more rapidly than the tributary valleys.12

When the ice eventually retreats, the floors of these tributary valleys are left at an elevation significantly higher than the floor of the main glacial trough, creating what is known as a hanging valley. Because these valleys join the main trough at such a discordant, elevated junction, postglacial streams flowing from the hanging valley must drop abruptly to reach the main valley floor, resulting in the formation of a waterfall. A well-known example of this phenomenon is Yosemite Falls in California.123