Why do tributary glaciers create hanging valleys?

Hanging valleys are created because of the difference in erosive power between a large trunk glacier and its smaller tributary glaciers.

In a glacier system, the main trunk glacier carries a much greater volume of ice than the smaller tributaries that feed into it. Because ice thickness is a dominant factor in the deepening process, the trunk glacier possesses a significantly greater capacity to erode its bed.12

As the main glacier rapidly carves out and deepens its path, the smaller tributary glaciers erode their own troughs at a much slower rate. Over time, the floor of the main valley is lowered far below the elevation of the tributary valleys. When the ice eventually retreats, these tributary troughs are left "hanging" high up on the steep walls of the main U-shaped valley. This discordance in elevation often results in the formation of waterfalls, such as Yosemite Falls, as postglacial streams flow from the mouth of the hanging valley down to the main valley floor.12