Forest Cover, Impervious-Surface Area, and the Mitigation of Urbanization Impacts in King County, Washington

Abstract

For decades, watershed urbanization has been known to have severe consequences on aquatic systems. Although the problem has been long articulated, solutions have proven elusive because of the complexity of the problem, the evolution of improving but still- imperfect analytical tools, and socio-economic forces with different and often incompatible interests. King County, Washington, has been a recognized leader in the effort to analyze and to reduce the consequences of urban development, but even in this jurisdiction the path has been marked by well-intentioned but ultimately mistaken approaches, compromises with other agency goals that thwart complete success, and imperfect implementation of the measures that ultimately have been adopted. The designation of ESA-listed species within the urban and urbanizing parts of the Puget Sound region has brought new scrutiny to all aspects of these watershed-mitigation efforts. Such increased attention is forcing a better articulation of the goals, the means, and the justification for mitigating the effects of urban development. This paper is one manifestation of that attention. The purpose here is to remind readers of the scientific framework for evaluating the consequences of urban development on aquatic systems; to review the history of surface-water management in King County as it relates to the analysis and mitigation of those consequences; and to evaluate the basis for a specific proposal, first explored almost a decade ago, to limit effective impervious areas in high-quality watersheds at or below 10 percent and to maintain forest cover above 65 percent.