Coulee

A coulee is a steep-walled ravine or valley, often dry, formed by glacial action or stream erosion. The term is most commonly used in the northwestern United States and Canada.

Coulees are typically characterized by their relatively narrow width and considerable depth, often cutting through basalt bedrock or other resistant geological formations. They can range in size from small gullies to large, canyon-like features.

The formation of coulees can occur through several processes:

• Glacial meltwater erosion: During periods of glacial melting, large volumes of water can be rapidly released, carving deep channels into the landscape. This is a common mechanism for coulee formation in areas that were formerly glaciated.
• Catastrophic flooding: Massive floods, such as those associated with the Missoula Floods in the Channeled Scablands of eastern Washington, can rapidly erode the landscape, creating a network of interconnected coulees.
• Stream erosion: Over long periods, the erosive power of streams and rivers can gradually carve out coulees, especially in areas with easily erodible soils or fractured bedrock.

The term "coulee" is also sometimes used more generally to refer to any deep, narrow valley or gully, even if not formed by glacial or fluvial processes. The specific geological context often determines the precise meaning and usage of the term.