In developed urban areas, naturally occurring depressions where storm water would pool are typically covered by impermeable surfaces, such as asphalt, pavement, or concrete, and are leveled for automobile use. Stormwater is directed into storm drains which may cause overflows of combined sewer systems or pollution, erosion, or flooding of waterways receiving the storm water runoff. Redirected stormwater is often warmer than the groundwater normally feeding a stream, and has been linked to upset in some aquatic ecosystems primarily through the reduction of dissolved oxygen (DO). Stormwater runoff is also a source of a wide variety of pollutants washed off hard or compacted surfaces during rain events. These pollutants may include volatile organic compounds, pesticides, herbicides, hydrocarbons and trace metals.