EFFECT OF TRIBUTARY INFLOWS ON THE DISTRIBUTION OF TRACE-METALS IN FINE-GRAINED BED SEDIMENTS AND BENTHIC INSECTS OF THE CLARK FORK RIVER, MONTANA

The effect of tributary inflows on metal concentrations in <63-mu m se diments and benthic insects was examined on two scales (380 km and <2 km) in a river impacted by mining. A dilution-mixing model effectively described large-scale dispersion of Gd, Cu, and Pb in the sediments o f the river. Input of metal from contaminated flood plains may introdu ce additional contamination in the middle reaches of the river. Intens ive sampling around the confluences of two tributaries showed that the re were significant, localized decreases in some metal concentrations immediately downstream of the inflows. Sediment metal concentrations 1 km below the inflows returned to values within the range predicted by the dilution-mixing model. Metal concentrations in benthic insects ex hibited spatial patterns similar to those of the sediments, indicating that biological exposures to metals are at least partially dependent on the physical processes controlling the dispersion of sediment-bound metals. Tributary inflows introduce variability in metal contaminatio n an different spatial scales that must be considered when assessing e cological risks in contaminated rivers. in addition to large-scale dil ution of contaminants, smatter areas of reduced metal exposure occur n ear tributary inflows. These may shelter metal-sensitive taxa from sev ere metal contamination in the mainstem.