The formation and transport of biogeogenic metal nanoparticles in mining-impacted environments is a developing concern because of their potential for greater distribution compared to larger particles. Legacy mine waste and metal-contaminated environments can produce soluble metal(loid) nanoparticles whose stability and transportability is determined by particle composition and environmental conditions. The Coeur d’Alene River Basin of northern Idaho, USA, is impacted by legacy mine waste—estimated 56 million tonnes of waste rock containing 800,000 tonnes of Pb and 650,000 tonnes of Zn were discharged into the Coeur d'Alene River and its tributaries during mining of argentiferous galena-sphalerite deposits. This legacy waste-disposal practice produced substantial contamination of floodplain sediments throughout the basin. For this study, monthly water samples and sediment cores were collected along the shoreline of a metal-contaminated lateral lake of the Coeur d’Alene River. Porewater was centrifugally extracted from an upper and lower layer of the sediment cores to evaluate the formation and stability of metal nanoparticles during seasonal changes from the spring snowmelt to the return of freezing temperatures in the late fall. Substantial concentrations of Fe, Pb, Mn, and Zn were present in 450-nm filtered porewater during each month, but concentrations decreased following spring runoff. Nanophase metals with an average diameter of 200 nm were present in the porewater of the upper and lower sediments throughout the year. Nanoparticles in the lower sediment porewater were consistently more stable (more negative ζ potential) than nanoparticles present in the porewater of the upper sediments. These sediment porewater differences correspond to changes in sediment S speciation as determined by synchrotron X-ray absorption spectroscopy of the residual sediments collected after centrifugation. The transport of these porewater metal nanoparticles was detected in the lake because of a change in the hydraulic gradient during a spring-to-summer transition that allowed the nanoparticles to move into the overlying water column of the lake. The presence of metal-carbonate and metal-sulfide minerals and seasonal redox conditions in the sediments of the lateral lake allow for production of stable and transportable metal(loid) nanoparticles throughout the annual hydrobiochemical cycle of this metal-contaminated environment in this northerly climate. This metal mobility because of nanoparticle formation creates an additional transport mechanism outside of free ion or sediment-bound metal transport.