A floodplain is the flat land next to a river that gets covered with water when the river overflows its banks during heavy rains or snowmelt. Picture a river running through a valley: the water normally stays within its channel, but during a big storm, it might spread out across the lower ground on either side. That spreading zone is the floodplain.

Rivers naturally deposit soil and nutrients across their floodplains when they flood, which is why these areas often have extremely fertile soil. Ancient civilizations like Egypt flourished along the Nile River's floodplain because annual floods enriched the land for farming. Many modern cities were also built on floodplains because the flat land made construction easier and rivers provided transportation and water.

However, building on a floodplain means accepting flood risk. When engineers design buildings or neighborhoods in these areas, they need to account for potential flooding through elevated foundations, flood walls, or drainage systems. Maps show 100-year floodplains (areas that have a 1% chance of flooding each year) and 500-year floodplains (areas that have a 0.2% chance of flooding each year). Despite the name, a “100-year flood” doesn't mean flooding happens exactly every hundred years. It could happen twice in ten years, or not at all for two centuries.