Stormwater runoff increases flooding. Although flooding is a natural process, stormwater runoff in very developed areas creates unnatural flash floods as a result of the sudden influx of water running off impervious surfaces like asphalt. This brings the risk of flood damage to structures farther from the stream. Indeed, new floodplain maps are being drawn for urban and suburban watersheds across the U.S. as increases in the size and frequency of floods are significantly enlarging floodplains.