A wadi, known also in the USA as an arroyo, is a river which is dry part of the year, with a bare river bed visible. A similar small part-time dry stream is called a wash.

The term wādī is Arabic, and very widely found in Arabic toponyms. Wadis are located on gently sloping, nearly flat parts of deserts; commonly they begin on the distal portions of alluvial fans and extend to inland sabkhas or dry lakes. In basin and range topography, wadis trend along basin axes at the terminus of fans. Permanent channels do not exist, due to lack of continual water flow.

Wadis have braided stream patterns because of the deficiency of water and the abundance of sediments. 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.

More Info: en.wikipedia.org