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Reducing flows in the Nechako River (British Columbia, Canada): potential response of the macrophyte community

French, T D, Chambers, P A
Canadian journal of fisheries and aquatic sciences 1997 v.54 no.10 pp. 2247-2254
biomass, data collection, equations, macrophytes, models, rivers, summer, watersheds, British Columbia
Approximately 50% of the Nechako River's flow was permanently diverted into another watershed in the early 1950s. Up to 50% of the remaining flow may be diverted in the future. To give insight as to how future and past flow reductions will/have affect(ed) macrophyte abundance, we first developed equations relating average summer channel speed to cross-sectional biomass and bottom cover from data collected at 26 sites. The average summer channel speed at each site was then estimated assuming flows (at Fort Fraser) of 408 m³ ·s⁻¹ (natural), 165 m³ ·s⁻¹ (1952-1990 average), and two future scenarios: 120 and 60 m³ · s⁻¹. We then used these estimates in our equations to compute abundance under the various flow regimes. Our models suggest that flow has little influence on macrophyte abundance in two fast-flowing reaches, which together account for 50% of the river's length. In contrast, the diversion was predicted to have increased biomass and cover by, on average, 66 g ·m⁻² and 15%, respectively, in a slow-flowing reach accounting for 20% of the river's length. Biomass and cover in this reach could increase by an additional 65 g ·m⁻² (or 240 g · m⁻²) and 9% (or 29%) if flows are reduced to 120 m³ ·s⁻¹ (or 60 m³ · s⁻¹).