Coulee, or coulée (/ˈkuːleɪ/ or /ˈkuːliː/)[1] 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
Frenchcouler 'to flow'.
The term is often used interchangeably in the
Great Plains for any number of water features, from
ponds to
creeks.
In southern
Louisiana the word coulée (also spelled coolie) 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.[2]
In the northwestern United States, coulee is defined as a large, steep-walled, trench-like trough, which also include
spillways and
flood channels incised into the
basalt plateau.[3]
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 100 m (330 ft) above the coulee floor.[3]
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, steep walls. "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 (i.e. the "Coulee Region").[4]
Aside from those formed by volcanic eruptions, coulees 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 southwestern
Canada. 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 (see
Driftless Area[5]). 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
riparianhabitats along streams in coulees and at the base of their walls.[6]
See also
Channeled Scablands – Landscape in eastern Washington state scoured by cataclysmic floods during the Pleistocene epoch
Grand Coulee – Ancient river bed in the U.S. state of Washington
^
abBaker, Victor (2010). Migon, Piotr (ed.). Channeled Scablands: A Megaflood Landscape, in Geomorphological Landscapes of the World. Springer. pp. 21–28.
ISBN9789048130542.