Regional risk assessment of the Puyallup River Watershed and the evaluation of low impact development in meeting management…

The Relative Risk Model (RRM) is a tool used to calculate and assess the likelihood of effects to endpoints when multiple stressors occur in complex ecological systems. In this study a Bayesian network was used to calculate relative risk and uncertainty (BN-RRM) in the Puyallup River Watershed. First, I calculated the risk of prespawn mortality of coho salmon. Second, I evaluated the effect of low impact development (LID) as a means to reduce risk. Prespawner mortality in coho salmon within the Puyallup watershed was the endpoint selected for this study. A conceptual model showing causal pathways between stressors and endpoints was created to show where linkages exist. The greatest risk of prespawner mortality was found in the urbanized risk regions with large amounts of impervious surface. The greatest risk reduction due to LID was observed in more developed regions, and implementing types of LID that are most effective in retaining and filtering stormwater during large storm events would be the most effective type. However, a great deal of surface area would have to be converted to LID to reduce the risk of stormwater impact to the coho fisheries. The structure of the BN-RRM also provides a framework for water quality-related and water quantity-related endpoints within this and other watersheds. The adaptability of using BNs for a relative risk assessment provides opportunity for the model to be adapted for other watersheds in the Puget Sound region.