A Risk-Based Approach to Environmental Flow Assessment

Habitat-based assessment methods used to determine environmental flow needs of rivers are riddled with uncertainties. Specifically, important uncertainties lie within quantifying (1) how the physical habitat within the river changes with flow, and (2) the physical habitat preferences of the organisms within the aquatic ecosystem. However, these assessment methods generally ignore all uncertainties in the analysis and produce a point estimate for the amount of usable habitat available to aquatic organisms at any given flow. When setting instream flow requirements, managers must be aware of the uncertainty in the habitat-flow relationship in order to make informed and precautionary decisions. In this research, we quantify the uncertainty in both the change physical habitat with flow and the physical habitat preferences of the organisms within the aquatic ecosystem. We develop a method that propagates these uncertainties through a commonly used assessment method in British Columbia, the British Columbia Instream Flow Methodology (BCIFM), to produce estimates of the habitat-flow relationship. Finally, we quantify the uncertainty in the habitat-flow relationship in terms of the risk of habitat loss as a function of flow. We apply this method as a case study to a steep, highly complex reach of the North Alouette River, BC. Results from this case study indicate that below a particular low-flow threshold, the probability of habitat loss increases exponentially. Additionally, we are more certain about the discharges at which high-magnitude habitat losses will occur compared to low-magnitude losses. It is important that this type of information be considered by managers when developing minimum flow requirements.