How do desert washes and wadis fill with water so fast?

A wash, arroyo, or wadi is a dry riverbed that may carry water only after heavy rain. Because desert soils are often hard, sparse, or already dry, they absorb little water, so rainfall can run off very quickly into channels. If a thunderstorm hits nearby mountains or uplands, floodwater can surge down the basin even when the desert floor itself is sunny and dry. These flows are dangerous because they can arrive with little warning and move rapidly through narrow channels. Desert wadis are a reminder that deserts can still experience violent flooding.