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Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

1120 SW 5th Avenue, Suite 1000, Portland, OR 97204

Hydrology and Infrastructure

Woods Creek culvertSteep slopes, soils that are slow to infiltrate rainfall, and impervious surfaces (such as streets) result in a flashy urban stormwater system. Impervious surfaces total about 1,500 acres, or 33 percent of the Fanno Creek Watershed. Impervious surface coverage is highest along major transportation corridors such as Beaverton-Hillsdale Highway, Capitol Highway, Interstate 5, and Barbur Boulevard.
 
Impervious surface cover in this range reduces rainfall infiltration, increases stormwater runoff volume and rate, and degrades stormwater quality. A 1991 study found that impervious cover greater than 10 percent reduces urban stream stability, resulting in unstable and eroding stream channels. These changes can degrade in-stream habitat and affect fish communities.
There is about 28 miles of open stream channel identified in the Fanno Creek watershed. An additional five miles of streams are in culverts or pipes.
 
Fanno Creek Mainstem

Development throughout the subwatershed has increased impervious surfaces and resulted in the loss of vegetation. Impervious surface coverage is highest along Fanno Creek, where commercial development and transportation routes predominate. These changes contribute to increased stormwater runoff volumes and velocities that can cause stream bank instability, undercutting, erosion, in-stream sedimentation, and channel incision.

 
Pendleton Creek

Single-family residential development predominates throughout the subwatershed. Impervious surfaces from development contribute to high stormwater runoff volumes and velocities that can cause stream bank instability and undercutting, erosion, in-stream sedimentation, and channel incision. These conditions can contribute to problems downstream in Fanno Creek.

 
Vermont Creek

Single-family residential development predominates throughout most of the subwatershed. Gabriel Park, a large open space tract, is located in the eastern portion of the subwatershed. Commercial multi-family development is concentrated in the southeast along SW Multnomah Boulevard, exhibiting the highest level of impervious surface cover in the subwatershed. Impervious surfaces contribute to high stormwater runoff volumes and velocities that can cause stream bank instability and undercutting, erosion, in-stream sedimentation, and channel incision. These effects are greatest in lower portions of the subwatershed and can contribute to problems downstream in Fanno Creek.

 
Woods Creek

Single-family residential development predominates throughout most of the subwatershed. Commercial land uses are concentrated in the south near Interstate 5, exhibiting the highest level of impervious surface cover in the subwatershed. Impervious surfaces contribute to high stormwater runoff volumes and velocities that can cause stream bank instability and undercutting, erosion, in-stream sedimentation, and channel incision. These effects are greatest in lower portions of the subwatershed and contribute to problems downstream in Fanno Creek.

 
North Ash Creek
Single-family residential development predominates throughout the subwatershed. Impervious surface cover is highest in the eastern portion of the subwatershed. Impervious surfaces contribute to high stormwater runoff volumes and velocities that can cause stream bank instability and undercutting, erosion, in-stream sedimentation, and channel incision. These conditions can contribute to problems downstream in Fanno Creek.
 
South Ash Creek
Single-family residential development predominates throughout the subwatershed. Impervious surface cover is highest along the Interstate 5 corridor in the southwestern portion of the subwatershed. Impervious surfaces contribute to high stormwater runoff volumes and velocities that can cause stream bank instability and undercutting, erosion, in-stream sedimentation, and channel incision. These effects are greatest in lower portions of the subwatershed and contribute to problems downstream in Fanno Creek.
 
Red Rock Creek

A mix of single-family, multi-family, and commercial land uses predominate in this subwatershed. Commercial land uses are concentrated in the north along Interstate 5 and multi-family residential development is located in the southwestern portion of the subwatershed. These areas exhibit the highest levels of impervious surface cover. Impervious surfaces contribute to high stormwater runoff volumes and velocities that can cause stream bank instability and undercutting, erosion, in-stream sedimentation, and channel incision. These conditions can contribute to problems downstream in Fanno Creek. Red Rock Creek is considered perennial, but it is likely that the tributaries east of Interstate 5 are seasonally intermittent.