REDUCING FLOWS IN THE NECHAKO RIVER (BRITISH-COLUMBIA, CANADA) - POTENTIAL RESPONSE OF THE MACROPHYTE COMMUNITY

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 cr oss-sectional biomass and bottom cover from data collected at 26 sites . The average summer channel speed at each site was then estimated ass uming flows (at Fort Fraser) of 408 m(3).s(-1) (natural), 165 m(3).s(- 1) (1952-1990 average), and two future scenarios: 120 and 60 m(3).s(-1 ). We then used these estimates in our equations to compute abundance under the various flow regimes. Our models suggest that flow has Littl e influence on macrophyte abundance in two fast-flowing reaches, which together account for 50% of the river's length. In contrast, the dive rsion 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 m(3).s(-1) (or 60 m(3).s(-1)).