A spatially extensive assessment of the environmental controls on mercury transport and bioaccumulation in stream ecosystems in New York and South Carolina was conducted as part of the U.S. Geological Survey National Water-Quality Assessment Program and included the determination of suvial transport of mercury and associated constituents during water years 2005-2009. (A water year extends from October of one calendar year to September of the next calendar year.) In the Coastal Plain region of South Carolina, the study area included the Edisto River and its headwater tributary, McTier Creek. In the Adirondack region of New York, the study area included the upper Hudson River and its headwater tributary, Fishing rook. Median concentrations of altered total mercury ranged from 1.55 nanograms per liter (ng/L) at the Hudson River site to 2.77 ng/L at the Edisto River site. The Edisto River site had the greatest mediana ltered methylmercury concentration, at 0.32 ng/L, and the Hudson River site had the least median altered methylmercury concentration, at 0.07 ng/L.