Effects of discharge/substrate stability relationships on the abundance, diversity, and distribution of aquatic insects were studied from 1986 to 1988 in three reaches of a small stream in the Manitoba escarpment. Substrate stability (percentage of streambed paving material at incipient motion) ranged from 80 to 20% at bankfull discharge. Eleven taxa of mayflies, 15 taxa of caddisflies, and 10 taxa of stoneflies were collected. All but a few taxa were present at all stations. Benthic insect densities were negatively correlated with increasing discharge and substrate particle movement at all stations. Regression models describing insect density in terms of substrate stability usually accounted for more variation than models describing density in terms of discharge. Insect density decreased up to 94% at the most unstable stream reach following summer and autumn spates when 8 and 12%, respectively, of the streambed paving material was at incipient motion wheres no reduction occurred in a stable riffle constructed nearby. Most insect species were highly resilient; densities and diversities returned to prespate levels during periods of low flow. Temporal and spatial aspects of substrate stability should be considered in stream insect studies, particularly in relation to disturbance.

A total of 3999 Trichoptera adults, represented by 8 families, 17 genera and 33 species was collected in emergence traps in 1983 and 1984 from five stations on the Ochre River, Manitoba (50°04' N, 99°48' W). Species composition for the two years was comparable, but as the result of a summer spate in 1984, abundance was only 40% of that in 1983.Species diversity by station was negatively correlated with substrate instability of the reach, whereas density per trap was negatively correlated with substrate instability and local factors such as sedimentation in some reaches following peak discharges. Analysis of historical peak discharge records indicated that relatively infrequent mid-summer spates had a detrimental effect on subsequent emergence of the Trichoptera fauna. The combination of spates and unstable stream bed substrate resulting from land use practices in the drainage basin have resulted in an impoverished caddisfly fauna in the Ochre River in comparison with other rivers in Manitoba.

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