Implications of legacy watershed disturbances for channel structure and salmon habitat availability under different low-flow levels: an analysis of 45 years of discharge–habitat relationships at Carnation Creek, British Columbia

In streams where water availability is limited, conservative flow ranges are often adopted by water managers to ensure that streamflow is available to meet the ecological requirements of aquatic organisms. However, a variety of natural and anthropogenic disturbances can influence stream channel morphology and in-stream wood characteristics through time, potentially altering the availability of habitat at a given flow level. Using a 2D hydrodynamic modelling approach incorporating 45 years of detailed channel morphology data from Carnation Creek, British Columbia, this paper examines relationships between legacy (forestry-driven) watershed disturbance, changes to channel morphology, and habitat availability for juvenile coho salmon (Oncorhynchus kisutch) under nine flow levels. Results indicate that substantial variability in the abundance of salmonid habitat is present through time, even when modelled flow levels are held constant. Additionally, trade-offs were observed between availability of habitat types as discharge increased. Finally, modelling results indicate that habitat availability is reduced following historical harvesting. These findings suggest that legacy watershed disturbances affecting stream channel form and function are worth considering when allocating streamflow.