Wadi

Wadi is the Arabic term traditionally referring to a valley. In some cases, it may refer to a dry (ephemeral) riverbed that contains water only during times of heavy rain or simply an intermittent stream.

Modern English usage differentiates a wadi from another canyon or wash by the action and prevalence of water. Wadis, as drainage courses, are formed by water, but are distinguished from river valleys or gullies in that surface water is intermittent or ephemeral. Wadis, cut by stream in a desert environment, generally are dry year round except after a rain. The desert environment is characterized by a sudden but infrequent heavy rainfall often resulting in flash floods. Crossing wadis at certain times of the year can be dangerous because of unexpected flash floods. Such flash floods cause several deaths each year in North America and many Middle Eastern countries.

Wadis tend to be associated with centers of human population because sub-surface water is sometimes available in them. Nomadic and pastoral desert peoples will rely on seasonal vegetation found in wadis, even in regions as dry as the Sahara, as they travel in complex transhumance routes.