Why are river valleys often V-shaped?

River valleys are often V-shaped because, during the initial stages of their development, rivers expend most of their energy in vertical entrenchment, or downcutting, into the landscape. This process is particularly pronounced in areas well above the regional baselevel, where the river cuts narrow, deep channels into the underlying geology.1

The V-shape is a result of this concentrated downward erosion, which is especially effective in rocks that resist lateral weathering. As the river continues to incise, it gradually smooths its longitudinal profile and eventually approaches its baselevel. Once this occurs, the river shifts its energy from vertical downcutting to lateral erosion, which progressively broadens the valley floor over time.1

It is important to note that this V-shaped morphology is distinct from the U-shaped cross-section characteristic of a glacial valley, which is formed when a glacier widens and deepens a pre-existing stream valley to minimize frictional resistance.2