Manganese (Mn) plays a critical role in river water quality since Mn-oxides serve as sorption sites for contaminant metals such as copper, zinc, and lead. Here, I measure and model annual changes in river water-groundwater interaction and Mn (aq) transport in an alpine streambed influenced by spring snowmelt (East River, Colorado). In field observations and models, oxygenated river water containing dissolved organic carbon (DOC) mixes with groundwater rich in reduced manganese in the riverbed. The depth of mixing is greatest during spring snowmelt when river discharge is greatest, leading to an influx of DOC, dilution of Mn (aq), and net respiration of Mn-oxides, despite an enhanced supply of oxygen. Conversely, a shift to upwelling conditions over the subsequent baseflow period allows for groundwater rich in Mn (aq) to mix with oxygenated river water in the shallow subsurface, resulting in net accumulation of Mn-oxides until the bed freezes in winter. To explore potential responses of Mn transport to different climate-induced hydrological regimes, I model three hydrograph scenarios (Historic, Low-snow, and Storm) for the Rocky Mountain region. In a warming climate with less snowpack and a longer baseflow season, Mn (aq) oxidation will be favored in the upper riverbed sediments over more of the year, which may increase the sorption capacity of the streambed for other metals.