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

Abstract

Abstract 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 estimate uncertainty (BN-RRM) in the Puyallup River Watershed. First, we calculated the risk of prespawn mortality of coho salmon. Second, we 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. A relative risk gradient was found throughout the watershed. The lowest risk was found in risk regions with the least urban development, and the greatest risk of prespawner mortality was found in the highly urbanized risk regions with the largest amounts of impervious surface. LID did reduce risk but only when implemented at high intensities within the urban watersheds. The structure of the BN-RRM also provides a framework for water quality- and quantity-related endpoints within this and other watersheds. The framework is also useful for evaluating different strategies for remediation or restoration activities. The adaptability of using BNs for a relative risk assessment provides opportunities for the model to be adapted for other watersheds in the Puget Sound and Salish Sea region. Integr Environ Assess Manag 2014;10:269–278. © 2013 SETAC Key Points The Bayesian network relative risk model (BN-RRM) estimated risk of prespawn mortality to Coho salmon in the Puyallup River watershed in a spatially explicit manner. The pattern of risk was that a greater risk existed as the watershed became more urban as it transitions through National Park, agriculture, suburbs, manufacturing and eventually to the Port of Tacoma. The effect of low impact development (LID) was successfully introduced into the model to examine the changes in risk. To successfully alter the pattern of risk, a substantial amount of area would have to be transferred to LID, especially in the high-risk regions of the watershed. The adaptability of the BN-RRM allows for both an evaluation of risk and an examination of how management alternatives alter the risk. The fundamental approach should be applicable to other sites in Puget Sound and to other scenarios.