Wadis are dry or have only intermittent or ephemeral streamflow. They range in size from small gullies, through large, broad valleys, to large, deep canyons. Consequently, the term covers a broad range of characteristics. For the present we are using the following classification:
The term "wash" is used colloquially in the southwestern U.S. and on maps to indicate an intermittent or ephemeral stream, e.g., "dry wash." In sedimentological terms, "wash" refers to the material deposited on the floors of gullies, arroyos, ravines, and canyons. It also forms more-or-less unconfined deposits on valley floors or outwash plains where it marks the location of former streamflow. A gully, the smallest of the drainageways, can be a feeder to a canyon, ravine, or arroyo or part of a drainage channel within a wash. An arroyo is a flat-bottomed channel with steep to vertical sides incised in semiconsolidated sediments. A ravine is a steep-sided, V-shaped erosion channel common to mountainous terrain. A canyon is the largest of the drainage valleys, but no dimensional boundaries between a ravine and a canyon have been established. A canyon is commonly cut into plateaus consisting of nearly horizontal rock layers. Canyons also occur in crystalline rock, where their cross-sectional profiles are less regular. A canyon can have steep sides and a V shape like a ravine. A canyon having near-vertical walls and flat floors, and that terminates upstream in a steep headcut is called a "box canyon." Inverted wadis are ridges that trace out former drainageways. The tops of the ridges were once the bottoms of channels. The surrounding ground has been removed by erosion. The ground under the channel was shielded from erosion by the protective nature of the material in the bottom of the channel. Inverted wadis are included here because, though ridges, their patterns in airborne and satellite imagery most often resemble drainage systems, which they once were. In monoscopic small scale imagery, it can sometimes be difficult to decide which of these two features the pattern represents.
Wadis may carry runoff from rain resulting from thunderstorms in distant headwater regions. These streams are generally loaded with silt and sand. Intermittent streams carry runoff from springs where ground water intersects the surface. Thus, these streams commonly contain high amounts of dissolved salts and may have a milky or green hue.
Many dry wadi courses on broad, open alluvial plains are floored by packed gravelly sediment and can provide good surfaces for cross-country movement. Where the washes are not flattened out and are aligned with the slope of a plain, they provide a "grain" to the surface and, depending on the roughness of the wash, travel can be easier parallel to the slope than across it. Channeled wadis such as arroyos, if choked with bouldery deposits or invading sand dunes, are unsuitable for cross-country movement. Steep-walled channels should be traveled with caution because of the potential for flash flooding. Such floods can result from rains in headwaters only a few kilometers or 100 km away. The flood front can move down a wadi as a high wall of muddy water traveling at more than 30 km per hour, depending on the gradient of the channel and the volume of the water. Near-surface water can often be found in braided wadi channels, particularly vegetated ones. The unconsolidated fill in wadi channels can usually be easily dozed and excavated. These materials can, however, be saturated and quick even beneath a dry surface (see Summary for Drainage Courses - Wadis - Arroyos ).
Both typical and inverted wadis have potential for cover and concealment and as sites for ambush and defilade. The gravel deposits in the channel can be sources of secondary projectiles when impacted with ordnance.
(common names are in bold) Dry wash, wady, ouadi, creek, gulch
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