Humans cause and exacerbate flooding due to urban development, destruction of natural wetlands and deforestation. Urban development causes water runoff into streams during heavy rain, and this runoff causes higher flood peaks and more frequent floods. Levee systems, instead of natural wetlands, cause more devastating floods during peak floods along river basins such as the Ohio, Mississippi and Missouri Rivers.
Runoff into urban streams empty into larger rivers, and these urban streams have limited capacities to move floodwaters properly. Floods are faster, higher and stronger in urban streams during heavy rainfall. Debris carried from city streets clogs waterways and increases the depth of floodwaters upstream of the clog. City planners can increase the number of channels through which the water flows to decrease the heights and speeds of floods.
Hard surfaces such as roads, parking lots, concrete, slanted roofs, drains and sewers move water quickly to more natural channels. When floodwaters pass over river banks, these natural systems erode and cause even more devastation. Rivers that do not have trees and natural vegetation flood and erode more quickly than those with plant life.
Decreasing levee systems, and increasing natural wetlands, lowers water levels along huge river basins during times of heavy rain. Floodwaters spread out over a wider area, but water levels are lower and slower compared to artificial, levee-based systems.