Implications of Land-Cover Change History for Monitoring Present and Future Ecological Condition in Nine Basins on the Urban Fringe of Seattle, Washington

Historic land use is a potentially significant factor determining present day watershed condition. Previous research has shown that historical land uses can have lasting effects on watershed condition that are manifest in present day hydrologic and water quality variables.  However, the relative importance of past land uses such as the extent and intensity of forestry or agriculture within a basin, remains poorly understood. As part of a larger project to monitor the effectiveness of King County’s land use regulations, we reconstructed land-cover conditions over approximately 100 years in nine small watersheds (80 – 1200 ha) in the Puget Sound region. We used these data to explore three questions: 1) how has forest cover changed overall within these watersheds; 2) how has forest cover changed within the riparian zone since 1936; and; 3) what land-cover changes are common to all watersheds and when and how do land-cover histories diverge?

We found that overall, the watersheds share a common history of forest-cover change, though the timing and extent of the change varied by watershed. The watersheds were primarily forested prior to 1900.  All the watersheds lost between 50 and 100% of their forest cover between 1900 and 1948 and existing cleared lands either transitioned back to forest or were converted to agriculture during this time. Between 1936 and 1948, all the watersheds except Taylor and East Seidel retained 40% to 60% of their riparian forest cover even though the watersheds overall were only 20 to 40% forested. The retention of forested buffers within the riparian zone potentially reduced the impacts of early logging on overall watershed condition and contemporary water quality.  Between 1948 and 1965, forest cover increased substantially in all watersheds, and by 1986, all watersheds were 60-100% forested. Since 1986, forest cover has declined slowly primarily due to conversion to rural residential land cover. In Cherry, Weiss and Tahlequah watersheds, development occurred primarily on previously forested lands without intervening agricultural land use. Comparatively, in Fisher, Judd and Taylor watersheds much of the early residential development occurred on previously agricultural lands. In addition, these latter watersheds developed to a greater extent than the other three treatment watersheds.

This analysis reveals potentially important variation in land-cover history among watersheds, which today have very similar land-cover characteristics. Identifying commonalities and variation in land use history is potentially critical for understanding present day conditions and the overall trajectory of watershed change in the future.