Wadi
A wadi is a river valley or a wet (ephemeral) riverbed that typically contains water only when heavy rain occurs, though wadis with year-round water flow do exist, such as Wadi Bani Khalid and Wadi Mawr.
Wadis are located on gently sloping, nearly flat parts of deserts; commonly they begin on the lowest portions of alluvial fans and extend to inland sabkhas or dry lakes. Permanent channels rarely exist, due to lack of continual water flow. Water percolates down into the stream bed, causing an abrupt loss of energy and resulting in vast deposition. Wadis may develop dams of sediment that change the stream patterns in the next flash flood.
Text adapted from Wikipedia, CC BY-SA 4.0. Photograph: Wikimedia Commons (see Commons), Wikimedia Commons.