Low Flows, Instream Flow Needs and Fish Ecology in Small Streams

Low flows in Canadian streams and rivers can occur in both summer and winter and can be stressful for fish and other aquatic biota. Low flows can cause a reduction in habitat availability, food production, and water quality and can accentuate the effects of river ice during the winter. Human demands for out-of-channel use of water during low flow periods have resulted in the development of a suite of tools for determining the instream flows needed to maintain desired ecological attributes (most often fish populations) of the stream. This paper reviews the impact of low flows on aquatic resources in small streams as well as instream flow methods and the empirical support for them and concludes that there remains substantial uncertainty in the prediction of impacts of flow reductions or diversions. Some of this uncertainty is due to a lack of understanding of the relationship between flow and fish populations, but much is probably due to site- and time-specific variation in how stream biota responds to habitat changes. A risk-based approach, that explicitly acknowledges the uncertainty in both the hydrology and biology, is needed for decision-making in water management.