The Bristol Bay watershed in southwestern Alaska supports the largest sockeye salmon fishery in the world, is home to 25 Federally Recognized Tribal Governments, and contains large mineral resources. The potential for large-scale mining activities in the watershed has raised concerns about the impact of mining on the sustainability of Bristol Bay's world-class fisheries, and the future of Alaska Native tribes in the watershed who have maintained a salmon-based culture and subsistence-based lifestyle for at least 4,000 years. The U.S. Environmental Protection Agency (USEPA) launched this assessment to determine the significance of Bristol Bay's ecological resources and evaluate the potential impacts of large-scale mining on these resources. The USEPA will use the results of this assessment to inform the consideration of options consistent with its role under the Clean Water Act. The assessment is intended to provide a scientific and technical foundation for future decision making; the USEPA will not address use of its regulatory authority until the assessment becomes final and has made no judgment about whether and how to use that authority at this time.