Stormwater runoff & impervious surfaces

Perhaps the most defining characteristic of urban streams is the increase in the amount and rapidity of stormwater or surface runoff to those systems. Impervious surfaces associated with urbanization reduce infiltration and increase surface runoff (Fig 16), altering the pathways by which water (and any associated contaminants) reach urban streams.

Common impervious surfaces include:

• Roads
• Parking lots
• Rooftops
• Driveways and sidewalks
• Compacted soils

How does stormwater runoff affect streams?

It alters natural hydrology, generally leading to more frequent, larger magnitude, and shorter duration peak flows.

It alters channel morphology, generally leading to changes such as increased channel width, increased downcutting, and reduced bank stability.

It alters in-stream hydraulics, affecting biologically important parameters such as water velocity and shear stress.

It disrupts the balance between sediment supply and transport, generally leading to increased sediment transport capacity and channel erosion.

It increases stream temperatures, due to the transfer of heat from impervious surfaces to stormwater runoff.

It increases delivery of pollutants from the landscape to the stream. Pollutants commonly found in stormwater runoff include:

• sediment

• nutrients
• pesticides
• wear metals
• organic pollutants
• oil and grease

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