A valley is a low area between hills or mountains typically with a river running through it. A valley is typically U-shaped or V-shaped.
A valley formed by flowing water is usually V-shaped. Rivers with steep gradients, like mountain ranges, produce steep walls and a bottom. In the lowest stretch of a river, where a slope approaches its base level, a valley collects sediment and can flood.
A valley carved by glaciers is normally U-shaped and resembles a trough. When the ice recedes or thaws, the valley remains, often littered with small boulders that were transported within the ice. Floor gradient does not affect the valley's shape, it is the glacier's size that does.