Surprising salmonid response to water diversion at four run-of-river hydroelectric projects in British Columbia

Abstract

  • Run-of-river (RoR) hydroelectric power is considered a renewable energy source with potentially fewer impacts on fish habitats compared to large reservoir-storage projects.

  • This study observes the abundance of rainbow trout (Oncorhynchus mykiss) in four streams in British Columbia using a before–after control–impact design.

  • Despite a 63% to 70% reduction in flow at diversion reaches, rainbow trout biomass increased by 35% to 157%.

  • Responses to water diversion varied by age class:

    • Adult biomass increased with reduced flows during the growing season.

    • Juvenile biomass increased during winter with reduced flows, higher stream conductivity, and increased summer flows.

    • Fry biomass increased with higher conductivity and temperatures.

  • Explores carrying capacity and growth/survival through a size–density approach.

Introduction

  • Importance of understanding how changes in flow due to hydroelectric projects impact river ecosystems and fish abundance.

  • Previous studies indicate harmful ecological effects from large dams on fish habitats, particularly salmonids.

  • The study focuses on RoR projects which divert water but maintain flow to downstream reaches.

Background

  • RoR projects typically generate less than 25 MW and significantly reduce flows typically to 10% or lower of the natural mean annual discharge (MAD).

  • This study analyzes four projects (Tipella, Lower Fire, Douglas, and Stokke) located within a 12 km radius in the Harrison watershed.

    • All projects constructed simultaneously, affecting similar small streams with steep gradients.

Methodology

Monitoring Design

  • The study used a before-after control-impact (BACI) design, monitoring:

    • Two years before project commencement (2006-2008) and five years during operation (2010-2014).

  • Each stream had an impact reach downstream of the diversion and a control reach upstream.

Data Collection

  • Collected water flow, temperature, water quality, invertebrate density, and rainbow trout density using closed-site electrofishing methods.

  • Additional measures of water quality included conductivity, dissolved oxygen, turbidity, and chemical analyses.

  • Invertebrates were sampled using drift nets in riffle habitats.

Analytical Techniques

  • Analyzed data using linear mixed-effects modeling to test for changes in fish density and biomass.

  • Examined key fixed effects (reach, period) and random effects (site nested within year).

  • Utilized size-density analyses to explore production across age classes relative to environmental conditions.

Results

Flow Regime Characteristics

  • Flows in the control reach were initially consistent with diversion reaches but changed over the monitoring period.

  • Compliance monitoring confirmed consistent instream flow requirements were met during project operation.

Rainbow Trout Response

  • Post-diversion:

    • Increased rainbow trout population density (11% to 115%) and biomass (35% to 157%), with notable increases in all age classes, excluding fry in Tipella Creek.

  • Flows during both growing and winter conditions were pivotal in shaping biological responses.

Age Class Differences

  • Responses varied; adult trout density correlated positively with lower mean growing season discharge.

  • Fry biomass was influenced by stream productivity and degree days above 5°C but not by flow metrics.

  • Juvenile biomass was affected by winter flows and required more stable conditions for growth.

Discussion

  • Positive fish responses to flow diversion at RoR projects differ from most documented negative responses in the literature.

  • Possible reasons for the observed increases in biomass could involve maintained high habitat suitability in steep streams, where natural habitats were preserved even at low flows.

  • Suggested that lower proportions of water diverted may lead to improved fish biomass and community structure.

Conclusion

  • The increase in rainbow trout biomass associated with flow diversion at RoR sites in BC presents a significant finding that contrasts with existing literature.

  • The results highlight the importance of hydrological management in balancing energy production with the need to maintain aquatic ecosystems.

  • Recommendations for further studies on long-term effects of RoR projects on fish communities and associated habitats.

Acknowledgements

  • Funding and support were provided by Innergex Renewable Energy and constructive feedback from various ecological and fisheries management professionals.

References

  • Extensive literature cited throughout the study focusing on hydrology, fisheries management, ecological assessment.

  • Documentation of previous studies suggests variations in fish population dynamics associated with flow management.