Carli Creek: A Reflection on Sustainable Industrial Stormwater Treatment 

In 2018, Herrera completed its 2-year project with Clackamas County to improve the water quality and surrounding habitat of Carli Creek. This unique project aimed to provide ecological uplift and long-term resiliency by seamlessly blending the large stormwater treatment area into surrounding ecotypes. With this, Herrera designed a regional constructed wetland complex to maximize the quantity of water treated by the industrial area, while continuing base flows and dynamic stream flows within Carli Creek. As this project enters year two of its vegetation establishment period, we reflect on our team’s approach and innovative solutions to improve the water quality of Carli Creek.

The Problem

Carli Creek’s watershed contains over 400 acres of industrial properties and major arterials with high-use industrial traffic. Originating from a stormwater pipe that drains a predominantly industrial basin, stormwater runoff containing high levels of pollutants discharged into Carli Creek and flowed directly into the Clackamas River, less than half a mile from several drinking water intakes. Before the project’s completion, stormwater monitoring data revealed pollution levels in the runoff to exceed standards for zinc, lead, copper, and E. coli. These stormwater pollutants along with hydromodification (the alteration of the natural flow of water) negatively impacted Carli Creek, resulting in severe degradation.

Carli Creek’s poor water quality also posed a risk to native fish inhabitants, such as cutthroat trout, sculpin, dace, Coho, steelhead, and Chinook salmon. Farming, weed dominated upland and riparian vegetation, and stream channel incision disturbed the habitat of these threatened and endangered species, emphasizes the importance of stormwater treatment and habitat restoration in Carli Creek.

Project Solutions

Based on the pre-construction conditions of Carli Creek, Herrera’s interdisciplinary team developed an approach to improve the creek’s water quality and habitat. To treat industrial runoff from over 400 acres of land, Herrera’s team designed a natural filtering system composed of a variety of constructed wetland cells. This integrative solution utilizes plants, soils, and microbes to slow down infiltration, capturing and removing pollutants from the industrial stormwater runoff, improving overall water quality. By the end of construction, over 70,000 native plants were installed to capture runoff pollutants.

To improve fish and wildlife habitat in Carli Creek, Herrera developed a solution to reduce stream erosion by adding diversity to the in-stream habitat, and creating a variety of diverse, native vegetation communities. Throughout the duration of this project, Herrera’s team restored 1,700 feet of Carli Creek with wood habitat structures, incorporating 83 wood structures to improve habitat for fish and wildlife. A variety of wood structures were included to enhance the creek including beaver dam analogs, grade control features, bank protection structures, and habitat structures. In addition, Herrera strived to integrate stormwater treatment and habitat improvement into the site through grading, invasive and non-native species removal, and planting a diversity of native species, enhancing the floodplain, and benefiting the entire watershed.

Project Design

With the goals of improving water quality and addressing the impacts of hydromodification on Carli Creek, Herrera designed a regional constructed wetland complex, integrated with a new backwater channel and floodplain bench of Carli Creek. This design also restored and enhanced native habitat throughout the 14-acre site, improving conditions within the creek channel, at the creek’s confluence with the Clackamas River, and in upland areas. The facility reconnects the creek channel with its floodplain, restores hydrologic and water quality function by promoting hyporheic exchange (the mixing of surface and subsurface water through porous sediment) and infiltration, and increases habitat diversity.

Herrera’s design includes a deep stormwater diversion pipeline to route the “first flush” away from the existing outfalls to the treatment area. The diversion storm line included construction or replacement of approximately 1,800 lineal feet of 18 to 24-inch diameter storm sewer, connections to existing structures, two flow diversion structures, and a hydrodynamic separator for pre-treatment. From the outfall, a log and boulder step pool structure conveys runoff into the initial detention cell.  The detention cell spills over into the three-cell wetland treatment terrace which discharges into an enhanced floodplain – creek terrace with backwater channel. The complex flow path allows for maximum treatment time within the facility before discharging into the enhanced habitat of Carli Creek.

Herrera developed a two-dimensional PCSWMM model to simulate the hydrology and hydraulic routing for the study basin to size the stormwater treatment facility, confirm the level of treatment that can be achieved, and evaluate the interactions between outflows from the proposed facility and stream flows within Carli Creek.


Following the completion of this project, Herrera assisted the County in preparing a site monitoring plan. Developed in 2019, this manual summarizes the basis of design for various project elements, site observations, and recommendations for ongoing monitoring and maintenance. In addition to maintaining this site, Herrera’s Carli Creek project has gone on to win awards from the National Association of Counties (NACO), Pacific Northwest Clean Water Association (PNCWA), and the Oregon Association of Clean Water Agencies (ACWA).