Why do deserts sometimes get flash floods?

Deserts often have soils and rock surfaces that absorb little water, especially after long dry periods. When thunderstorms or monsoon bursts do arrive, rain can fall faster than the ground can soak it up, causing rapid runoff into washes, wadis, and dry riverbeds. Because vegetation is sparse, there is less interception and fewer roots to slow the water. That is why flash floods are one of the most dangerous hazards in deserts, even when annual rainfall is very low.