Stormwater Runoff

In urban and suburban areas, paved and developed surfaces prevent rainwater from soaking into the ground. Instead, rainwater quickly runs into storm drains, which carry it directly to the nearest stream.

Here are some of the problems caused by stormwater runoff in urban and suburban areas:

  • 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.Urban stormwater runoff often creates muddy streams as churning, fast-moving waters pick up sediment. Note how turbid the main channel of Rock Creek is, after a D.C. storm.
    Photographer: Sam Sheline, courtesy of NatureServe
  • Stormwater runoff increases water pollution. The stormwater brings with it harmful salts, excess nutrients, bacteria, herbicides, pesticides, heavy metals, hydrocarbons (from asphalt sealant), oil and grease, and litter. Pollutants contaminate or kill aquatic creatures, and harm wildlife and humans who harvest food from fresh, estuarine, and ocean waters.
  • Stormwater runoff decreases available groundwater. When rain lands on impervious surfaces, it is diverted to the nearest stream rather than soaking into the ground to become groundwater. Decreased groundwater results in streams and tributaries having lower flows (or no flow) in dry weather—reducing habitat for aquatic creatures or raising the water temperature beyond what they can handle. Decreased groundwater also challenges the moisture requirements of natural communities that flourish on floodplains and other low areas.
  • Stormwater runoff increases erosion in stream channels. The high velocity and volume of increased stormwater runoff erodes stream banks and exposes tree roots.
  • Stormwater runoff increases turbidity. The fast-flowing water, sometimes already muddied from construction site runoff before it reaches the stream, picks up more sediment as it erodes stream banks. The extra sediment suffocates aquatic creatures, prevents sunlight from reaching aquatic plants, and clogs up the protected spaces between rocks in shallow riffles that shelter fish eggs, juvenile fish, and aquatic insects.