Flash Floods and Ephemeral Desert Rivers

Deserts are defined by scarce rainfall, yet paradoxically they are also home to some of the most sudden and violent flooding on Earth, because the rain that does fall typically arrives as brief, highly localized, and extremely intense convective thunderstorms rather than as the gentle, sustained precipitation typical of wetter climates. Desert soils compound this danger: prolonged dryness often produces a hardened, compacted, sometimes cemented surface layer, in many regions a calcium-carbonate crust called caliche, that absorbs water far more slowly than ordinary soil, so that even a short, intense downpour generates rapid surface runoff rather than infiltration. This runoff concentrates quickly into dry stream channels, called wadis in Arabic-speaking regions and arroyos in Spanish-speaking ones, which remain empty and often invitingly walkable for months or years at a time before transforming, within minutes, into fast-moving torrents carrying water, mud, and debris.