The effect of current velocity and sediment on the drift of the mayfly Ephemerella subvaria Mcdunnough

SUMMARY. Experiments conducted in an artificial stream showed that significantly more nymphs drifted from an inorganic substrate at a mean current velocity of 28.5 cm s−1 than at 18.5 cm s−1. Drift density, however, was not affected. Disproportionately large numbers of nymphs drifted while current velocities were being increased from 18.5 to 28.5 cm s−1. Both drift numbers and drift density were greater in turbid water, after the addition of large amounts of inorganic sediment, than under clear‐flowing conditions during dark periods but not in the light. The interaction of increasing current velocity and sediment levels resulted in a significantly greater number of drifting nymphs under lighted conditions. Minor spates which do not seriously disturb the stream bed may initiate significant increases in macroinvertebrate drift.