The Trouble with Runoff

Natural vs. Urban Hydrology

Back before human development took over, much of the Tualatin Valley was wetlands that absorbed and filtered rain.  The headwaters in the West Hills and the Coast Range were forested with hemlock, fir and cedar that caught the rain and held it until it evaporated or infiltrated into the groundwater system.  Virtually none of the rainwater falling ran over the surface into streams.  Streams were fed by springs and seeps connected to the groundwater system.

Now in our increasingly urban environment, with rooftops, parking lots, streets and driveways, up to half of rainwater runs over ground and through drain pipes into our neighborhood streams, and far less ends up recharging the groundwater system.  During a storm, the flow in an urban stream can increase over one hundred-fold in a matter of hours.   (Click to see hydrograph). A storm can quickly change urban streams from a trickle to a gushing torrent. Runoff from storm events carry the pollutants of urban life: oil from cars, metals from brake linings, pesticides from lawns, moss killers from roofs, pet feces, garbage and a host of other pollutants.  High flows cause erosion that destroys habitat and suspends sediments that can deplete dissolved oxygen. See USGS Fanno Creek Study.

With so little stormwater infiltrating into the ground, our streams suffer in the dry season.  Historically, the groundwater system would feed streams with cooling flows in the summer.  With groundwater depleted, urban streams are reduced to a trickle and heat up.  Low flows and high temperatures are compounded by low oxygen levels.  The streams that once supported young steelhead and cutthroat trout in the summer can no longer support these fish.

Growth in Impervious Area – Paved Paradise

Clean Water Services has estimated in 2001 that 28% of land within the Urban Growth Boundary of the Tualatin Basin was impervious area.  The Bronson Creek watershed increased from 14.5% impervious area (465 acres) in 1994 to 22.4% (715 acres) in 2001.  Of this impervious area 54.5% is dedicated to car habitat (streets, driveways, parking lots) and the remainder being other human habitat (buildings and sidewalks). 

Studies by the University of Washington indicate that impacts biological communities of streams are detectable when impervious area of the watershed reaches 5-8%.  They also found that macroinvertebrate communities are moderately to severely impaired when impervious area reaches a threshold of 10-15%.

To learn more about pollutants in urban runoff, view the video After the Rain: Urban Runoff, produced by Oregon State University Extension.