Why are most valleys drained by rivers?

Most valleys are drained by rivers because they are primarily created by the erosion caused by those same rivers. While some valleys, such as rift valleys, are formed by tectonic activity where the depression exists before the river, the overwhelming majority of valleys—including canyons and gorges—share a genetic bond with the rivers that flow through them.1

Rivers act as agents of landscape evolution by lowering the level of erosion through degradation. As rivers flow, they expend energy to incise the land, adjusting their paths toward a baselevel, which is typically sea level. During the initial stages of development, rivers focus their energy on vertical entrenchment, creating narrow, deep valleys. Over time, as they reach an elevation closer to their baselevel, they expend more energy laterally, which progressively broadens the valley floor. Consequently, the size and shape of a valley are directly related to the size and erosional power of the river that occupies it.231