As part of its comprehensive stormwater planning and management effort, the City of Mercer Island (City) implemented subbasin scale drainage improvements to facilitate silt removal and erosion control, and to minimize the input of degraded runoff to Lake Washington, the receiving waterbody.

In 2008 stormwater, sediment, and macroinvertebrate sampling was conducted in five Mercer Island streams as a continuation of the monitoring program established in previous years as described in the 2006 annual report (King County 2008). In March 2009 the underlying questions driving the monitoring effort were redefined and the monitoring program was revised. Baseflow monitoring for conventionals, bacteria, and herbicides in all basins was added to the program to characterize water quality during summer low flow conditions. This report presents results from field and laboratory investigations conducted in 2008 through 2010.