Synopsis: This paper articulates and summarizes relationships between (i) forest cover and (ii) impervious surface area as watershed indicators. These indicators strongly influence hydrology, stream channel stability, bank erosion, sedimentation, water quality, fish and other aquatic habitats. The area of focus in King County, western Washington State, has experienced rapid development. The paper provides an excellent background on technical principles, empirical monitoring results, and models applied to study these issues.
Key empirical results summarized include the relationships between upstream impervious area and fish habitat quality (Figure 6), and channel stability, peak flows and land use (Figure 9). One key finding was that less empirical data are available on the direct correlations between forest cover and stream conditions than for watershed imperviousness and stream conditions. The “evidence” with regards to forest cover has been primarily based on observed correlations of channel instability to modelled hydrologic analyses exploring relationships between peak flows and forest cover.
The analyses, including extensive empirical studies spanning 20 years, were used to support limiting effective impervious areas in high quality watersheds to at or below 10%, and to maintain forest cover at a minimum of 65%, in order to effectively mitigate the impacts of urbanization and development on watersheds. These thresholds mark an observed transition to degraded stream conditions.
Another key finding was that riparian buffers can also reduce the magnitude of urban impacts; however, these cannot fully mitigate the impacts of upstream development in the watershed.
|journal||Journal of the American Water Resources Association , 33, no. 5 (1997): 1077-1090|