Mercury Maps is a tool that relates changes in mercury air deposition rates to changes in mercury fish tissue concentrations, on a national scale. The tool utilizes a reduced form of accepted mercury fate and transport models applied to watersheds in which air deposition is the sole significant source. The Mercury Maps model states that for long-term steady state conditions, reductions in fish tissue concentrations are expected to track linearly with reductions in air deposition watershed loads. The model utilized in this project is a reduced form of the IEM-2M and MCM models used in the Mercury Study Report to Congress (MSRC) (US EPA, 1997b), whereby the equations of these models are reduced to steady state and consolidated into a single equation relating the ratio of current/future air deposition rates to current/future fish tissue concentrations. Mercury Maps is designed to work only with watersheds in which air deposition is the sole significant source of mercury. A key step in the project then is to identify, and eliminate from the analysis, watersheds in which mercury sources other than air deposition, such as gold mines and chlor-alkali facilities, are present and contribute loads that are significant relative to the air deposition load to that watershed.