The creek and its floodplain can be considered as a unit…
The floodplain is the relatively flat area within which a creek or river moves and on which it overflows. Unobstructed, the water flow erodes the outer banks of creeks and deposits sediment on their inner banks. Thus, creek and river channels do not remain in the same location forever, but move around the floodplain over time.
Floods exaggerate these effects. They increase creek bank erosion, rapidly carve new channels, and deposit large amounts of gravel and sand downstream. In urban areas, increased volume of stormwater can make swollen creeks spill over onto the floodplain. Flood peaks in urban areas tend to have higher levels and faster velocities and also occur more frequently than in natural streams. Together, these factors increase scouring and erosion of urban creek banks.
The building of homes and businesses on floodplains have limited the ability of the floodplain to hold floodwaters. Expensive flood mitigation schemes are required to provide a degree of flood immunity. However such schemes also destroy the natural waterway corridors, replacing them with straight, often concrete-lined channels to carry the greater volume of water. It is a far better strategy to control the filling and development of the floodplain, in the first place.