A wash results when fast-moving water from flash floods fills low-lying channels on the desert floor. The scouring action of sand, rocks, and plant debris carried by the water cuts a meandering path, or wash. A desert wash is usually dry but will fill quickly with runoff during heavy rains.
Moisture is retained longer in washes so the vegetation is greener and more abundant than in the surrounding desert. Trees such as palo verde (Cercidium floridium), and smoke tree (Psorothamnus spinosus), often grow in or near washes. Their hard seeds need the scouring action of rushing, churning water to crack and germinate.
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Also see:
Desert Wash Habitat
Geology - Stream Channel Formation
Desert wash