Instream flow studies in perspective

The Instream Flow Incremental Methodology (IFIM) has been subject to criticisms, including its apparent imprecision, inability to predict discharge‐biomass relationships, lack of independence of hydraulic variables, and omission of predation/competition as variables in assessing the dynamics of aquatic populations and communities. This paper addresses criticisms of the methodology, stressing three themes. First, the development of IFIM to its present form is described. The goal of the method is to relate biotic values in equivalent terms to those used to estimate other beneficial uses of water. As such the engineering concepts of hydraulic simulation and suitability criteria play a strong role in the model. Previous studies suggest that IFIM appears to perform defensibly in coldwater systems but less well in more complex coolwater and warmwater systems. Second, the strengths of IFIM are considered and the type of environmental of IFIM are considered and the type of environmental problems it is suited to address are described. Research suggests that biotic responses vary dramatically as certain threshold discharges are approached and it is suggested that biomass predictions are inappropriate with current versions of IFIM. Its greatest utility is shown to be in assessing the impacts of water resources development on habitat availability for aquatic organisms. Third, the limitations of IFIM are presented; those that appear to have merit and those that arise from misapplication or misunderstanding of the methodology. We suggest that suitability criteria be developed on a site specific basis and include depth‐velocity dependent functions. The added predictive power by incorporation of coefficients of biological interactions to this management model is probably not justified by the expense required to obtain the data. As a tool, IFIM maximizes generality and precision at the expense of ecological reality but this does not detract from its utility to analyse water resource issues.