Stormwater, often referred to as stormwater runoff, is the rainwater and snowmelt that runs off of developed surfaces, such as streets, lawns, and roofs. (EPA Facility Stormwater Management). In a natural setting, the ground absorbs precipitation and it is filtered by plants and soil before reentering the water cycle. In developed areas, impervious surfaces convey pollutants that can seriously impair streams, rivers, and the wildlife that live within. (Rainwater Management at CWS - Clean Water Services) In addition to precipitation washing over streets, roofs, lawns, and parking lots picking up trash, sediment, and other contaminants, stormwater runoff contributes to bank erosion, downstream flooding, and riparian habitat destruction. As Washington County continues to develop, and as our knowledge of contaminants of emerging concern grows, comprehensive stormwater management is critical to maintaining a healthy Tualatin River watershed.
Who manages stormwater in Washington County?
Stormwater management is the process of ensuring that the runoff is clean by the time it reenters the water bodies in a watershed. In Washington County, Clean Water Services (CWS) is responsible for managing the flow of stormwater runoff, a system known as stormwater conveyance, and monitoring the quality of the water entering the Tualatin River and its tributaries. CWS works with city staff and other organizations to implement a variety of solutions, from maintaining the stormwater conveyance system (over 500 miles of storm sewers and pipes, plus drainage ditches, detention ponds, and water quality facilities!) and developing construction and design standards, to educating the community and protecting vegetated corridors (riparian buffer zones) between new development and creeks and wetlands. (Rainwater Management at CWS - Clean Water Services)
How does stormwater impact the Tualatin River?
Although CWS and local municipalities work hard to comprehensively manage stormwater runoff, gaps remain. Current gaps are primarily the legacy of past infrastructure developments that were constructed before modern management practices were established. These developments, often uphill from the Tualatin River, are chock full of impervious surfaces and lack runoff mitigation resources, such as swales or rain gardens. This leaves rain and snowmelt to run down the pavement and eventually create new intermittent streams that carry pollutants directly into the river. Along the way, these streams can cause significant damage to public and private properties in the form of erosion and flooding.
One particularly damaging effect of erosion are headcuts. When a stream of stormwater runoff meets the river, the flow causes the bank to erode to the point of creating a vertical or near vertical drop. As the amount of runoff increases, this erosion, known as a headcut, continually migrates further up the runoff stream. This migration causes channel bed and bank instabilities, worsens erosion, and increases the sediment load delivered to the river.
What’s in store for stormwater management in Washington County?
Stormwater management presents a complicated challenge to utility providers, municipalities, and developers. Successful management requires solutions that look both forward—regulating new construction to prevent runoff from negatively impacting a watershed—and backward—fixing or mitigating issues caused by past absent or inefficient policies. It also requires a landscape-level approach: addressing the cumulative effect of all development in a region, as opposed to development by development.
In Washington County, Clean Water Services is leading the development of a regional stormwater management plan. In June 2025, in coordination with King City, Tigard, and the county, it released the South Bull Mountain Regional Stormwater Draft Concept Plan. During the public comment period that followed the release of this draft, Tualatin Riverkeepers submitted a comment in support of CWS’ effort. Overall we are highly encouraged by CWS’ regional approach and look forward to seeing this approach applied across the watershed. Read our comment letter.
