Ecological streamflows with geomorphic functions
Geomorphic processes control channel morphology and bed material characteristics. These processes are important to maintain the stream habitat which aquatic species utilize for different purposes such as rearing, feeding and reproduction. Alteration of geomorphic functions is associated with changes in streamflow. The aggradation of channel units and the accumulation of sediment in their substrate, which has negative effects on many biological requirements of aquatic species, is one common consequence. Several environmental flow methodologies have recommended streamflows based on providing a certain range of water depths and velocities (i.e. hydraulic habitat) for fish. However, these methodologies have not examined the link between the biological requirements of aquatic species and the full range of streamflows that maintain channel morphology and bed material (i.e. geomorphic habitat). In addition, the sampling strategies and hydraulic tools used by these methodologies have not addressed the effect of flow variability or different geomorphic habitat scales. In this study, a methodology called the Functional Unit Method (FUM) was developed. The methodology has two components: a conceptual model that structures and integrates knowledge of the biological requirements of aquatic organisms with geomorphic functions, and the streamflows required to sustain suitable channel morphology and bed material characteristics with focus on gravel-bed streams; and a stream management framework component that can direct managers to appropriate stream management or process-based restoration activities. The conceptual model of FUM was applied to identify ecological flows with geomorphic functions to maintain pool and riffle habitats in a gravel-bed stream in Southern Ontario. Techniques to quantify streamflows with geomorphic functions were developed and tested in the Credit River at Cataract in Ontario using a combination of sediment entrainment and direct calibration methods. The Credit River at Cataract is a gravel-bed stream with pool-riffle morphology, with seasonal flow regime consisting of high flows in the spring and low flows with occasional stormflows in the summer. The techniques were used successfully to quantify flows that can remove superficial sediment from pools during the spring and fall to maintain summer and winter habitat use for fish, and remove sand from riffles in the early summer to maintain summer habitat use for fish and benthic invertebrates. The quantification of streamflows with geomorphic functions demonstrated the importance of the direct calibration component of the technique to identify potential limitations of the sediment entrainment model for a particular stream. The tools developed complement existing environmental flow techniques but enable identifying of streamflows critical to maintaining geomorphic habitat and the effects of hydro-geomorphic processes on habitat dynamics.