Lower Columbia River Field Guide to Water Quality Friendly Development
  IntroductionWhy Is This Important?Techniques and ExamplesIntegrated Site ExamplesResources

The Problem
A Solution

The Stormwater Management Problem
Earth’s water continuously circulates from the atmosphere to the land, from there into groundwater and surface water, and then back to the atmosphere. This is called the water cycle, or hydrologic cycle, and is key to understanding stormwater impacts.

In natural (undeveloped) conditions, rainfall infiltrates slowly into the ground. Natural biologic processes cleanse the water as it moves through vegetation and soil and into groundwater. Because most Pacific Northwest rainstorms are not large enough to fully saturate the soil, only a small percentage of rainwater flows over the surface as runoff. What does become runoff usually travels at a slow, meandering pace. Particles and sediments settle out along the way, ridding the water of impurities before it flows into rivers and streams.

The Hydrologic Cycle: Effects of Impervious Land on Runoff Quality and Quantity

In pre-development conditions, a large percentage of precipitation infiltrates into the soil. A small percentage remains on the surface as runoff.  

In post-development conditions, impervious surfaces greatly reduce infiltration. A much larger percentage of precipitation becomes surface runoff.

Diagram courtesy of NEMO

For Metro’s graphic version of development’s impact on the hydrologic cycle click here.


When impervious area in a watershed reaches 10 percent, stream ecosystems begin to show evidence of degradation. Coverage of more than 30 percent is associated with significant degradation. Developed urban areas typically have impervious surface coverage of well over 30 percent.

Development drastically alters these conditions. Impervious surfaces such as buildings, roads, parking lots, and sidewalks prevent rain from soaking into the ground. There is less vegetation to soak up, store, and evaporate water. As a result, stormwater runoff over the land surface greatly increases, even during small rainstorms. This alteration of the water cycle has significant impacts to rivers and streams:


Increased runoff volume and speed cause flooding and erosion and destroy natural habitat.

Because less water infiltrates into the ground, less groundwater recharge occurs. This reduces steam base flows, which is harmful to fish and aquatic organisms.

Impervious surfaces retain heat, which increases runoff temperature during warm weather. This in turn raises the temperature of the receiving waters, negatively impacting aquatic life.

Stormwater runoff –also called surface water runoff—is the largest source of water pollution affecting Oregon’s rivers, lakes, and streams.

Stormwater runoff collects oil, fertilizers and pesticides, metals, chemicals, sediments, bacteria, and other pollutants and carries them into rivers and streams.


Traditional stormwater management basically seeks to “get rid of” runoff. Gutters, drains, and pipes collect runoff from impervious surfaces and convey it to discharge points. Large volumes of untreated stormwater rapidly discharge into natural water bodies.

Protecting our rivers and streams is vital for a great number of uses, including fish and wildlife habitat, recreation, and drinking water. As the region continues to grow and develop, the harmful effects of excessive stormwater runoff will only increase—unless we do things differently.


The Problem
A Solution

A Stormwater Management Solution
Across the region and country many new developments are utilizing different approaches that significantly lesson stormwater impacts. Capturing, treating, and infiltrating stormwater on site is the new focus. Stormwater management becomes an integral element of site and building design, rather than a consequence or afterthought of development. It’s an approach with both strong environmental benefits and great possibilities to enhance developed or developing properties.

Water quality friendly stormwater techniques involve one or more of the following strategies:

Reduce paved surfaces. This strategy promotes stormwater infiltration into the ground and decreases surface runoff. It can be accomplished by:

  • Retaining the natural landscape – protecting and encouraging trees and open space.
  • Minimizing pavement – through approaches like narrow driveways, parking lot spaces, and travel lanes.
  • Using permeable surfaces – such as permeable pavement, turf block, and gravel instead of concrete.

Disconnect impervious areas. Any impervious area that drains into the stormwater system is a “connected impervious area.” These areas can be “disconnected” by directing the runoff elsewhere. Most commonly residential downspouts are routinely directed to landscaped areas or rain barrels.

Intercept stormwater. Capturing rainwater before it comes into contact with an impervious surface. Trees, ecoroofs and roof gardens all intercept rainfall.

Detain and infiltrate stormwater. Storing stormwater to allow it to soak into the ground or move more slowly into the storm system. Planter boxes, infiltration basins, swales, soakage trenches, and drywells all provide infiltration.

Filter stormwater. Filtering out pollutants as stormwater moves through vegetated facilities such as planter boxes, swales, filter strips, infiltration basins and sand filters.

These processes realize significant benefits. Runoff volume, speed, and temperature are reduced. Groundwater recharge and stream base flows are replenished. The quality of runoff entering rivers and streams is improved. The result is that development achieves a better balance with the natural water cycle and becomes water quality friendly.

Added Value
In addition to improving the health of the region’s water resources, techniques that control stormwater runoff on site can offer many other advantages:

  • Improve air quality by filtering out air pollutants.
  • Reduce air temperature through shading and decreased impervious surfaces.
  • Provide wildlife habitat.
  • Add aesthetic appeal and increase property value.
  • Reduce energy costs by insulating and shading buildings.
  • Collect water for reuse, reducing the amount of water we use.
  • Help meet the challenge of allowing for development while preserving water quality which directly impacts a region’s livability.