A weight of evidence approach in environmental assessment includes the use of biomonitor organisms to measure biologically available contaminant concentrations and lethal and sublethal responses in an exposure, dose, and response framework. Corbicula australis was assessed as a test species for metal toxicity using in situ river sediment exposures at 4 locations in the Molonglo River (New South Wales, Australia), which has a legacy of sediment metal contamination, following 8 decades of mining in its upper reaches. A sediment metal contamination gradient was evident from 12.5 km to 47 km downstream of the mine, as follows: zinc (851–130 mg/kg) > lead (104–7 mg/kg) > copper (31–5 mg/kg) > cadmium (2–0.3 mg/kg). Exposed C. australis accumulated the following metals in tissue: zinc (1358–236 μg/g) > copper (24–20 μg/g) > cadmium (4.7–0.7 μg/g) = lead (4.2–1.8 μg/g). Biomarker responses showed increased sublethal impairment with increased tissue metal concentrations. Total antioxidant capacity was mildly impaired, with corresponding increased lipid peroxidation and lysosomal membrane destabilization at the higher tissue metal concentrations. Corbicula australis proved to be an effective biomonitor organism for sediment metal assessment, as it is able to accumulate metals relative to sediment concentrations and showed a pattern of increased sublethal impairment with increased tissue metal concentration. It is recommended as a suitable species for incorporation into local freshwater monitoring and assessment programs. Environ Toxicol Chem 2017;36:709–719.