City of Seattle, Seattle Public Utilities, Delta Plant Communities 1988-2007, Cedar River Municipal Watershed, King County, WA

Seattle Public Utilities manages the Cedar River Municipal Watershed and reservoir, Chester Morse Lake, to provide drinking water for 1.6 million residents in the greater Seattle area. The Cedar and Rex rivers are the two largest tributaries to Chester Morse Lake and flow over broad, low-gradient deltas. The deltas have mostly fine sediments, sinuous low-flow channels, and an extensive wetland complex with aquatic, herbaceous, shrub, and forest components. Delta plant communities were mapped in 1988, 1996, and 2007 using aerial photography. Plant communities were ground-truthed and boundaries and classification of polygons were corrected where errors were evident. Plant communities were classified into major structural classes, including herbaceous, shrub, deciduous forest, mixed deciduous/conifer forest, and conifer forest. A system of permanent plots was established on the Cedar and Rex river deltas and measured in 1988, 1996, and 2007. Transects comprised of sample plots every 25 meters were surveyed for herbaceous and shrub cover. An additional transect was established in the floodplain of the Cedar River through mixed deciduous and conifer forest to measure tree diameter at breast height and species. This package is complete, and the data were analyzed to evaluate the potential for future adverse impacts to delta plant communities resulting from changes to the reservoir operating regime.