Sediments of the Wabigoon River, Ontario, are polluted with Hg from an industrial source at Dryden. Downstream from Dryden the river flows first through Clay Lake and then through Ball Lake. Hg, Cu, Zn, Cd and Fe concentrations in sediments of both lakes are correlated with organic C, but Hg content per unit concentration of organic C is higher, and increases more steeply with Organic C, in Clay Lake than in Ball Lake; in contrast, the Cu, Zn, Cd, and Fe concentrations per unit concentration of organic C are nearly the same in both lakes. These results substantiate the conclusion that the Hg originated at a point upstream from the lakes and tended to accumulate in Clay Lake on its way downstream, whereas the other metals were leached into the two lakes from diffuse sources in the surrounding terrain. Accumulation of river-borne organic pollutants in Clay Lake was also demonstrated. Application of the methods employed to the general problem of recognising and locating element anomalies is discussed.