This data summary report, which does not include any statistically-based interpretation or discussion, has been prepared as a means of communicating results of this sampling exercise to all interested parties as soon as possible. The anticipated audience would include government agency staff associated with water quality management and the development of the Lake Ontario Lakewide Management Plan (LaMP), as well as researchers examining the ecosystem effects of toxic substances (persistent trace organics) within the Lake Ontario drainage basin. This study was undertaken as a screening level exercise to identify those watersheds (if any) exhibiting cumulative evidence of contaminant sources within their entire drainage area and to illustrate the general relationship between land use and water quality. It was not designed to identify specific contaminant sources within watersheds. The distribution of these data is intended to encourage and accelerate additional data synthesis and analysis by federal and provincial agency staff and the research community. It is anticipated that subsequent analysis will include a more rigorous statistical examination of the relationships among land use, flow conditions, and contaminant concentrations, along with a comparison of water quality findings with biomonitoring data. The potential application of these data to the estimation of loadings will also be examined.