Coulee, or coulée is a term applied rather loosely to different landforms, all of which refer to a kind of valley or drainage zone. The word coulee comes from the Canadian Frenchcoulée, from the French wordcouler meaning "to flow". The term is often used interchangeably in the Great Plains for any number of water features, from ponds to creeks. In southern Louisianathe wordcoulée originally meant a gully or ravine usually dry or intermittent but becoming sizable during rainy weather. As stream channels were dredged or canalized, the term was increasingly applied to perennial streams, generally smaller than bayous. The term is also used for small ditches or canals in the swamp. In the northwestern United States, coulee is defined as a large, steep-walled, trench-like trough, which commonly are spillways and flood channels incised into the basalt plateau.
Types and examples
The dry, braided channels formed by glacial drainage of the Scablands of eastern Washington, such as Grand Coulee and Moses Coulee. Moses Coulee and Lenore Canyon, lower Grand Coulee, have hanging valleys, where pre-flood tributaries enter the coulees at least above the coulee floor.
In the western United States, tongue-like protrusions of solidified lava, forming a sort of canyon.
In Wisconsin, it is applied to valleys. These valleys tend to have high, steepwalls. "Hollow" is used as a synonym, often for the smallest of such valleys. The term is also applied to the greater La Crosse, Wisconsin metropolitan area.
Aside from those formed by volcanic eruptions, they are commonly canyons characterized by steep walls that have been shaped by erosion. These types of coulees are generally found in the northwestern United States and southwesternCanada. In the American west, rapid melting of glaciers at the end of the last ice age caused catastrophic flooding which removed bedrock by massive down-cutting erosion, forming deep canyons. Some coulees may be seasonally dry or contain small streams, however these small misfit streams do not have the magnitude of force necessary to form such expansive erosion. In Wisconsin, they are the product of nearly a half million years of erosion, unmodified by glaciation. The loose rocks at the base of the wall form what are called scree slopes. These are formed when chunks of the canyon wall give way in a rockslide. Left alone, the valleys are often woodland, with the ridgetops transitioning into tallgrass prairie when not turned into pasture or used for row crops. Coulees provide shelter from wind and concentrated water supplies to plants which would otherwise struggle to survive in the xericsagebrush steppe. Trees are often found in riparian habitats along streams in coulees and at the base of their walls.
