Though uncommon, when precipitation comes to the desert it can do so in torrential downpours sending a flash flood churning down dry streambeds known as a wash, arroyo or wadi depending on region. Salt encrusted soils result as water rapidly evaporates in the desert climate. A playa forms as an ephemeral lake in a low region of closed drainage. Permanent lakes and perennially running stream are rare in deserts. Many rivers are exotic streams, rivers whose head waters lie in a wetter region and the majority of which flows through a desert.
Alluvial fans are another prominent feature of many desert regions. Alluvial fans are fan-shaped alluvial deposits generally found where a mountain stream runs on to a flatter surface at the front of a mountain system. Mountain streams carrying a heavy stream load loses their kinetic energy as they flow out on to the flat plain depositing alluvium. Alluvial fans are quite common in arid regions where water is lost to evaporation and infiltration into coarse surface material when the stream exits the mountain front. Deposition of the sediment cause the channel to migrate horizontally depositing alluvium. Through time the channel migrates back and forth depositing sediment until a fan-shaped deposit is formed. A bajada forms when several individual alluvial fans merge into one broadly sloping surface.