In streams with limited water availability, conservative flow ranges are often adopted by water managers to ensure that streamflow is available to meet the ecological requirements of aquatic organisms alongside other uses. An assumption underlying this approach is that conditions governing channel hydraulics relevant to fish habitat remain relatively constant through time. However, watershed and fluvial processes related to natural and anthropogenic land surface disturbances can influence stream channel morphology and in-stream wood characteristics, with a high potential for altering habitat availability through time, even if streamflow does not vary.
This presentation describes the results of a study examining how changes in channel morphology and wood abundance produce differences through time in five flow-based habitat metrics relevant to juvenile Coho. We also review connections between forest land use- driven watershed disturbance, changes to channel morphology, and habitat availability under nine flow levels commonly used across many in-stream flow needs (IFN) methodologies. We will address two questions: 1) Is geomorphically-induced habitat variability important in relation to annual variability in seasonal streamflow; and 2) Can temporal differences in habitat be attributed to forest land use-related watershed disturbance?
To address these questions, we make use of a 2D hydrodynamic modeling approach incorporating 45 years of detailed channel topography and wood data from Carnation Creek, B.C. Results indicate that substantial variability in habitat abundance occurs through time even under constant flow conditions. Trade-offs were also found to exist between habitat metrics (e.g., pool-based variables and areas of higher velocity flow) as discharge increases. Finally, our results indicate that legacy watershed disturbances lead to a significant reduction in habitat availability for pool-based variables. These findings suggest that the assessment of watershed disturbance state and consideration of channel functionality are important to explicitly consider when balancing human and aquatic water needs.