river valley, especially a dry (ephemeral) riverbed that contains water only during times of heavy rain

A Wadi is usually a valley, sometimes the bed of a river. The river that is part of the wadi only has water after heavy rain. Staying in a wadi can be a danger to life. This is because the water in a wadi can rise very quickly. Canyon is another name for it, used in other countries.

A wadi in Oman
A wadi in the Negev, Israel