A floodplain is a generally flat area of land next to a river or stream that stretches from the banks of the river to the outer edges of the valley. Floodplains experience flooding during periods of high discharge, and they are an important storage site for sediments during their transport from where they are generated to their ultimate depositional environment. Floodplains support diverse and productive ecosystems, and they are characterized by considerable variability in space and time, which in turn produces some of the most species-rich of ecosystems.

Floodplains consist of two parts: the main channel of the river itself, called the floodway, and the area adjacent to the floodway, called the flood fringe. Floodways can sometimes be seasonal, meaning the channel is dry for part of the year. Floodplains are natural flooding outlets for rivers, but people, agriculture, and businesses on floodplains are always at some risk. Managing development of floodplains is a critical responsibility for regional and urban planners.

Floodplains provide numerous flood loss reduction benefits as a result of their unique natural functions. Rivers and streams shape floodplain topography and influence riparian habitats and riverine ecosystems. The physical characteristics of the floodplain shape water flows and can provide flood loss reduction benefits such as excess water storage and slowing runoff. Flood storage is particularly important in urban areas where even small floods can cause severe damage.

Floodplains are dynamic natural systems characterized by processes such as periodic flooding, erosion, and deposition. They provide a wide range of benefits to local communities, including fish and wildlife habitat protection, natural flood and erosion control, surface water quality maintenance, groundwater recharge, and biological productivity. Flood waters can be deceiving in terms of their depth and how quickly they move, and it is not safe to walk or drive through flood waters since as little as six inches of water can cause you to lose your balance, and two feet of water can sweep your car away. Flood waters may carry silt, raw sewage, oil, or chemical waste, and if your well has been flooded, you should assume that the well is contaminated.