Downcutting streams are streams that are actively eroding their beds and deepening their valleys. This process is driven by the force of water flowing over the stream bed, which picks up sediment and carries it away. Downcutting streams are typically found in areas with steep slopes and high rates of erosion, such as mountains and canyons.

A youthful stage of valley evolution is a stage in which the valley is still being actively formed by downcutting streams. The valley is typically narrow and V-shaped, with steep slopes and a high rate of erosion. The valley floor is often covered with sediment and debris, and there may be waterfalls or rapids present.

Downcutting streams are an important part of the process of valley evolution. They help to create the shape of the valley and provide a pathway for water to flow from the mountains to the sea. Downcutting streams can also be a source of sediment, which can be deposited downstream and form new landforms.