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Stormwater Action Monitoring Status and Trends Study of Puget Lowland Ecoregion Streams: Evaluation of the First Year (2015) of Monitoring Data

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The long-term goal of this study is to monitor how stream health changes over time in Puget Lowland streams as the area urbanizes and stormwater controls are implemented more broadly. The first round of monitoring in 2015 evaluated the current condition of wadeable streams within urban growth areas (UGAs) and outside UGAs representing a range of development conditions and impacts of stormwater runoff on small streams. The study questions were: 

• What is the status of Puget Lowland ecoregion stream health within and outside UGAs? 

• What are the major natural and human stressors impacting stream health?  

• How do the results of this study compare to other stream monitoring programs?

• What monitoring parameters should be carried forward for SAM small stream monitoring in the future, and at what timing and frequency? 

Many of the stream health measures, such as fecal coliform bacteria, total phosphorus, benthic index of biotic integrity (B-IBI), indicated poorer condition in streams within UGAs compared to streams outside UGAs. For example, 82 percent of stream length within UGAs was in poor condition based on B-IBI scores, while 31 percent of stream length outside UGAs was found to be in poor condition. Key stressors identified included watershed and riparian canopy cover, stream substrate characteristics, and nutrients. Watershed and riparian canopy cover were found to be the most important stressors to B-IBI at the regional scale. This suggests that canopy cover protection and recovery (reducing impervious surface) could lead to substantial improvements in B-IBI scores.

Comparisons of SAM streams data to other Puget Lowland stream monitoring programs were made for B-IBI scores and parameters representing water and sediment quality and stream habitat measures. Variability in results among programs was attributed primarily to differences in study designs, spatial sampling extent, and differences in methods. 

Recommendations for SAM small stream monitoring in the future included two options: a minimum change scenario that maintains the two UGA strata but modifies the list of target parameters (e.g., eliminate monthly water quality sampling; add continuous stage measurement) and a second option that recommends a modification in the design to focus more specifically on the gradient of urbanization (relatively undeveloped to highly urbanized) that is more broadly captured by the two UGA strata.

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