Freshwater mussel populations in the Powell River have been in decline over several decades, with no indication of recovery. Identification of the cause(s) of mussel decline and continued suppression is a critical step toward improving water quality and restoring mussel populations in the Powell River. Toxicity from major ions and/or trace elements released from coal-mine discharges is hypothesized as a candidate but experimentation is needed to test that hypothesis. This study generated critical data on concentrations of major ions and trace elements, along with their spatial and temporal variation, which will enable accurate characterization and thus replication of environmentally-relevant exposure conditions for such experiments. We sampled water-column (surface) and substrate interstitial (pore) water seven times over the course of one year at nine locations in Powell River spanning 157 river kilometers. We found major ions and trace elements exhibited considerable longitudinal and intra-annual seasonal variation, with highest concentrations observed for most measured constituents in late summer/fall concurrent with low river discharge, and in river segments nearest coal-mining and other land uses that can increase erosion potential (urbanization, agriculture, forestry). Concentrations of trace elements were generally low relative to water-quality criteria in filtered surface and pore water with the exception of selenium, which regularly exceeded 40% of its chronic criterion but rarely exceeded it. In contrast, substantial amounts of trace elements were associated with interstitial fine sediment particles, with all toxic elements except arsenic having potential to exceed their chronic criteria if they were to become bioavailable, especially copper and lead. Although most major ions were well-correlated between surface and pore water, trace elements were less so, rendering prediction of pore-water concentrations from surface-water concentrations less likely. These results demonstrate the potential unreliability of estimating potential toxic exposures to benthic organisms from surface-water concentrations alone, and underscore the need for further research of all environmental compartments that may serve as sources and routes by which aquatic life, especially freshwater mussels, may be exposed to trace elements. The data generated by this study will facilitate such research by enabling characterization of realistic exposures to trace elements, to include how their spatial and temporal variation influence such exposures.