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- Drift at the control site was relatively constant and did not exhibit strong diel fluctuations. Drift at the impact sites appeared to respond to daily discharge fluctuations.
- Changes in discharge may have a greater impact on macroinvertebrate drift than absolute flow levels.
- Mean daily drift biomass was significantly higher during double-peaking.
- Drift increases were sustained for approximately 2 hours and only for 30–60 days despite ongoing hydropeaking. Modeling suggest that both drift increases and the ephemeral nature of increases were related to patterns in vegetative export and not changes in the density or distribution of benthic macroinvertebrates. The rate of vegetative export likely declined through time because of natural seasonal senescing initiated by decreased light levels and cooler temperatures at the onset of double-peaking in October.
- Drift increases were proportional to peak magnitude, with drift biomass peaking during the rising limb of the hydrograph. For every increase of 10 m3·s−1, biomass was predicted to increase by 0.81 mg. Drift biomass increased 400% relative to pre-experimental conditions during the experimental increase of peak flow.
- Both within- and among-day drift hysteresis appeared related to patterns in vegetative export.
- Increases in macroinvertebrate drift were not associated with detectable reductions in benthic densities, while inconsistent and modest taxa richness reductions were observed.
- Gut fullness for both brown and rainbow trout increased significantly following periods of hydropeaking. There was no significant differences in the relative abundance of diet composition between flow regimes.
Twofold daily variation in discharge resulted in:
- a >10-fold increase in drift concentrations of benthic invertebrates associated with pools and detritus (i.e. Gammarus lacustris and Potamopyrgus antipodarum).
- In contrast, drift concentrations of sessile blackfly larvae (Simuliium arcticum), which are associated with high-velocity cobble microhabitats, decreased by over 80% as discharge doubled.
- Drift concentrations of Chironomidae increased proportional to discharge.
Drift of all four taxa was positively related to benthic density.
- Drift concentrations of Gammarus, Potamopyrgus and Chironomidae were proportional to benthic density.
- Drift concentrations of Simulium were positively related to benthic density, but the benthic–drift relation was less than proportional (i.e. a doubling of benthic density only led to a 40% increase in drift concentrations).
Twofold daily variation in discharge associated with hydropeaking was the primary control on within-day variation in invertebrate drift concentrations. In contrast, benthic density, which varied 10- to 1000-fold among sampling dates, depending on the taxa, was the primary control on invertebrate drift concentrations over longer timescales (weeks to months).
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