Storm water runoff poses significant threats to water quality in Washington

Trees reduce stormwater runoff by intercepting rainfall in their canopies and roots. Photo: Guy Kramer
Trees reduce stormwater runoff by intercepting rainfall in their canopies and roots. Photo: Guy Kramer

Storm water runoff – the rain that falls on streets, driveways, rooftops and other developed land — is one of the most widespread challenges to water quality in Washington state. It carries oil, grease, fertilizers, soaps, and waste from pets and failing septic systems into streams and other bodies of water.

DNR has set a goal to clean up and restore Puget Sound, because even the clean water that originates in the upland forests we manage can become polluted as it flows through urban and suburban areas.

One of the best ways to mitigate the negative impacts of urban and suburban storm water runoff is to reduce how much of it ends up in natural waterways. Trees and shrubs are part of the solution because they help detain storm water on-site, in addition to slowing its flow and reducing erosion. October is an excellent time to recognize the many benefits that trees provide, including reduction and filtration of storm water runoff, because they:

  • Reduce storm water runoff by intercepting rainfall in their canopies where it is later re-released into the atmosphere.
  • Slow down runoff rates and reduce pollutants by absorbing storm water through their roots.
  • Store pollutants and transform them into less harmful substances.
  • Create healthy soil conditions that allow rainwater to filter into the soil so that less flows down streets, sidewalks, gutters, and storm sewers.

Here’s what you can do to help the trees in your community do a better job of filtering and managing storm water runoff:

  • Decrease the amount of hard surfaces (like concrete) that block water from infiltrating into the soil.
  • Advocate for more trees and vegetation in your community.
  • Preserve healthy, established trees through proper maintenance and care.
  • Minimize the clearing of trees and vegetation ­– you’ll also help reduce soil erosion.
  • Avoid over-fertilizing or over-watering your trees and lawn.
  • Route excess storm water to a natural retention area, such as a vegetated area with healthy soil which can filter out pollutants, reduce runoff rates and volumes, and prevent soil erosion.
  • Retrofit parking areas and other locations with extensive hard surfaces with new plantings of trees, shrubs and other plants — strategically located, they can intercept storm water and allow it to filter into the ground.

Trees, of course, are not the only means to address the challenges of storm water runoff, but here at DNR, we believe they are a big part of the solution, which is why we support Gov. Jay Inslee’s decision to proclaim October as Urban and Community Forestry Month.

For more tips and ideas, visit the Washington State Urban and Community Forestry Program, which operates with support from the US Forest Service.

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