Flash floods happen within hours of heavy rain, dam or levee failure, on small watersheds and in canyons. They are the leading cause of flood deaths because the water is fast, powerful and arrives with little warning — and because people underestimate it, especially in vehicles.
River (riverine) flooding builds over days as rain and snowmelt drain into larger rivers. It is more predictable, with crests that can be forecast in advance, but it inundates wide floodplains and can keep rising long after the rain stops.
Urban flooding occurs where pavement and drainage can’t absorb intense rain; water ponds in streets, underpasses and basements even far from any river. Coastal flooding comes from storm surge and high tides pushing seawater inland, often alongside a hurricane.
Many disasters combine types: a tropical system can bring surge at the coast, flash floods inland, and days-later river flooding downstream — which is why FloodWatch pairs naturally with storm tracking.