coulee

An upwards view of section of the wall protruding to the canyon. There is a marsh in the foreground.

A view through a coulee, with steep but lower sides, and water in the bottom.

Drumheller Channels in the Columbia Basin of Washington

A view upward into a coulee in the Oldman River valley in Lethbridge, Alberta

Coulee (or coulée) is 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 French coulée, from French word couler meaning "to flow".

The term is often used interchangeably in the Great Plains for any number of water features, from ponds to creeks.

An alternate meaning for this term exists in southern Louisiana where it identifies a large, paved channel for water runoff, but this derives from its original usage in reference to a creek, which was then paved to improve urban drainage.

Types and examples

1. The dry, braided channels formed by glacial drainage of the Scablands of eastern Washington, especially that of the Grand Coulee and the dam named for it.
2. The furrowed moraines channeling rain runoff in the area east of the Coteau du Missouri in the western United States and western Canada at the base of the Rocky Mountains.
3. In the western United States, tongue-like protrusions of solidified lava, forming a sort of canyon.
4. In Wisconsin, it is applied to smaller sometimes intermittent tributary streams in the watershed of the Upper Mississippi River, north of the Wisconsin River and as far up as the Saint Croix River. 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, rather much like "Twin Cities" is applied to Minneapolis-Saint Paul. The term has also been incorrectly applied to the whole of the Driftless Area, when in fact the Coulee Region excludes portions of the Driftless, while including portions which are outside of it.

In some parts of Louisiana coulees are not concreted but rather sheer sided large ditches that collect smaller ditch runoff.

Geomorphology

Aside from those formed by volcanic eruptions, they are generally deep steep-sided ravines formed by erosion, commonly found in the northwestern United States and southwestern Canada. In the American west, coulees in general were originally formed during the rapid melting of the glaciers at the end of the last ice age. Some coulees are dry for most of the year; others may contain small streams.

In Wisconsin, they are the product of nearly a half million years of erosion, unmodified by glaciation (see Driftless Area^[1]). 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 have a hard time surviving in the sagebrush steppe. Trees are often found next to streams in coulees and at the base of their walls.

References

See also

• Grand Coulee
• Channeled scablands