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Study Area


The study area is a complex and rapidly evolving landscape on the edge of a thriving metropolitan region. The Chehalem Mountains, on which the study area is centered, divide metropolitan Portland from the North Willamette Valley—Oregon’s most productive agricultural region. For generations these uplands have provided ecosystem services to a dozen communities in the surrounding valleys that contribute to the Portland metropolitan region. In recent decades these hills with superb climate and views have been discovered by viticulturists, Christmas tree growers, commuters and wealthy homebuyers. The population of surrounding counties has more than doubled since 1990. And though farm acreage declined by 2%, farms increased in number by 27% over this same period, reflecting the transition from commercial farms to hobby farms. Late summer streamflows are now low to non-existent, and some prime agricultural areas are closed to new groundwater rights. Runoff is a function of winter rain and area geology is not conducive to underground storage. Tualatin Valley Water District, Clean Water Services, and municipal partners have developed a plan to raise the height of the main supply reservoir at enormous cost, while half of Yamhill County’s 10 cities face periodic supply shortages. More than 20% of the Tualatin and Yamhill basins’ streams have been designated water quality impaired, with temperature the most pervasive problem followed by bacteria and phosphorous. Key 303d parameters include bacteria, temperature, and dissolved oxygen. Other indicators of degradation in this region include severe loss of some native habitats, including more than 90% of oak woodlands, and ESA-listed aquatic and terrestrial species. Little is understood about the extent of provision of the full suite of ecosystem services by this area to the surrounding populations, or the ability to support additional economic growth. Divided among three county jurisdictions, the Chehalem Mountain region has no cohesive community of its own, and developed without any cogent land use plan. Now, these uplands are at risk of losing their capacity to provide vital ecosystem services to the region before that reality is even recognized. In order to facilitate use of a multiscalar approach to capture the full effects of change on ecosystem services, a broad study area has been defined that includes the adjacent Yamhill and Tualatin River watersheds. Local watershed councils maintain the most comprehensive inventories of ecological data. Recently, the encompassing Willamette River Basin has been the focus of study on impacts from anticipated land use change and climate change independently of one another.