- 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).
Our results are counterintuitive and show that more natural steady flows reduced growth rates of juvenile humpback
chub compared with fluctuating flows when both treatments occurred within the same year. Daily growth rates during steady flows of 2009 and
2010 were 0.05 and 0.07 mm/day slower, respectively, than fluctuating flows those same years, despite similar water temperatures. Juvenile
humpback chub also grew more slowly during steady flows that occurred in the same season. During the summer, juvenile humpback chub grew
0.12 and 0.16 mm/day in fluctuating flow regimes in 2009 and 2010, respectively, and only 0.07 mm/day in the experimental steady flow
regime in 2011, despite higher water temperatures.
Comparisons:
- 2009: Fluctuating summer, steady fall
- 2010: Fluctuating summer, steady fall
- 2011: Steady flows summer and fall (equalization)
Findings:
- Seasonal mean daily growth rates of juvenile humpback chub in the Colorado River were lower during steady flows than fluctuating flows.
- Mean daily growth rates were also lower when steady flows followed fluctuating flows within the same year.
- Growth was faster during fall (0.14mm/day) compared with summer (0.07mm/day) when flow conditions were steady in both summer and fall.
- Growth during summer 2011 was lower than during the two previous years when fluctuating hydropeaking flows occurred even though temperatures were higher.
- Growth of juvenile humpback chub in the Little Colorado River was relatively high in 2010 (0.22mm/day) and did not decline in September and October as it had in previous years. This may be related to the active monsoon season in 2010.
- Flow conditions more strongly influence growth than water temperature across the range of flow treatments and temperatures observed during 2009–2011.
- Fall growth was faster in the mainstem Colorado River than in the Little Colorado River during two of the three study years (2009 & 2011).
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