Runoff is water from rain or melting snow that flows over land surfaces into streams, rivers, lakes, and storm drains rather than soaking into the ground. Impervious surfaces like pavement, roofs, and compacted soil increase runoff by preventing infiltration. Excessive runoff causes flooding, erosion, and water pollution as it carries contaminants from surfaces into waterways. Stormwater management—through retention ponds, permeable pavement, green infrastructure, and storm sewers—controls runoff to prevent flooding and protect water quality. Climate change increasing precipitation intensity makes runoff management more challenging. Municipal engineering departments design drainage systems to handle runoff while protecting environment and property.