Sampling of benthos at 166 stations in the lower two-thirds of the St. Marys River in 1974 and 1975 yielded a total of 9 mayfly and 20 caddisfly genera. Densities of mayflies (primarily the pollution sensitive Hexagenia) ranged from 0 to more than 2,000/m2 and caddisflies (primarily Polycentropus) from 0 to 744/m2 in individual samples. No mayflies were collected in a 20 km section of the river extending from below Sault Ste. Marie, Ontario, to the head of Lake George – an area previously identified by the International Joint Commission as having been “degraded” by industrial wastes. In three sections of the river progressively farther downstream from the degraded section, the average numbers of mayfly nymphs increased from 36 to 264 to 457/m2; and in three other non-degraded sections of the river, the range was 127 to 341/m2. Densities of caddisfly larvae were low (4/m2) in the degraded section of the river that was devoid of mayflies and ranged from 7 to 1017m2 in non-degraded sections. Average densities for all stations combined were 220/m2 for mayflies and 34/m2 for caddisflies.
