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

Abstract

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).