A suite of air quality and watershed models was applied to track the ecosystem contributions of mercury (Hg), as well as arsenic (As), and selenium (Se) from local and global sources to the San Juan River basin in the Four Corners region of the American Southwest. Long-term changes in surface water and fish tissue mercury concentrations were also simulated, out to the year 2074.Atmospheric mercury was modeled using a nested, spatial-scale modeling system comprising GEOS-Chem (global scale) and CMAQ-APT (national and regional) models. Four emission scenarios were modeled, including two growth scenarios for Asian mercury emissions. Results showed that the average mercury deposition over the San Juan basin was 21 µg/m²‑y. Source contributions to mercury deposition range from 2% to 9% of total deposition prior to post-2016 U.S. controls for air toxics regulatory compliance. Most of the contributions to mercury deposition in the basin are from non-U.S. sources. Watershed simulations showed power plant contributions to fish tissue mercury never exceeded 0.035% during the 85-year model simulation period, even with the long-term growth in fish tissue mercury over that period. Local coal-fired power plants contributed relatively small fractions to mercury deposition (less than 4%) in the basin; background and non-U.S. anthropogenic sources dominated. Fish-tissue mercury levels are projected to increase through 2074 due to growth projections for non-U.S. emission sources. The most important contributor to methylmercury in the lower reaches of the watershed was advection of MeHg produced in situ at upstream headwater locations.