This term is used to indicate pockets of water left in drainage channels or wadis during the dry season; the dimension and the duration of immersion vary from one guelta to another and from one region to another. They are temporary but can persist through very dry winters depending on runoff, water flow, and/or groundwater. They can occupy meanders left by permanent rivers (cf. ox-bow lakes of north Europe) as well as in temporary wadis. In the arid and steppe areas, the gueltas occur in lowland rivers and their deltas where flow becomes greatly reduced such as by channel blockage by sand dunes. In the Moroccan Sahara, these formations often correspond to oases; Gueltat Zemmour in Moroccan Sahara near Smara is a typical example.
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