Acid mine drainage (AMD) is acidic, metal-rich water produced from the oxidative dissolution of sulfide minerals exposed to air and water during mining. AMD can continue to contaminate water resources years after mining ends, damaging affected ecosystems and requiring costly reclamation. AMD-derived metals can be transported through water as dissolved ions, nanoparticles, and larger suspended particles. Although these species differ in toxicity and mobility, most AMD studies either do not include or do not differentiate between certain phases. The objectives of this study were to examine metal speciation and transform in an AMD-contaminated stream and determine the unique concentration-discharge behaviors (C-Q) of different metal species. Here, we examined metal speciation along the length of a stream that mixed with AMD contaminated groundwater and treated, alkaline water from an AMD-treatment system. Dissolved and nanoparticulate Fe concentrations spiked where AMD-contaminated groundwater upwelled into the stream, but decreased downstream as nanoparticulate aggregates of Fe-oxides and other Fe-containing minerals settled out of the water column. Streambed sediments contained high concentrations of Fe-oxides, pyrite, and Fe-sulfates. At the watershed outlet, nanoparticulate Fe concentrations decreased relative to dissolved Fe at times of higher discharge, indicating unique C-Q behaviors depending on metal speciation. Our results indicate that geochemical processes, such Fe-oxide precipitation, along the stream help sequester most contaminant Fe in the watershed. This work will help address a knowledge gap regarding the understanding of metal transport though streams and has implications for policy regarding the effectiveness of AMD treatment systems.