Guelta

A guelta (or qalta or galta or agelmam), from Arabic:قلتة which means a lower level of height in ground between rocks which holds water, is a pocket of water that forms in drainage canals or wadis in the Sahara. The size and duration will depend on the location and conditions. It may last year-round through the dry season if fed by a source such as a spring. When rivers (wadis) dry up there may remain pockets of water along its course (c.f. ox-bow lake). In Western Sahara, gueltas correspond with oases.

Map of Guelta d'Archei Lake, Chad

Some examples include Guelta d'Archei in Chad and Timia in Niger.