Riverine thermal regimes
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Riverine thermal regimes are an important but overlooked component of environmental flows. Dams and other human activities can strongly modify the natural thermal regime of rivers by altering water temperatures, often decreasing temperatures downstream. Five key challenges to incorporating thermal regimes into environmental flow assessments are: 1) improving understanding of natural thermal variability, 2) quantifying the degree dams alter thermal regimes, 3) relating thermal changes to ecological impacts, 4) developing thermal criteria, and 5) implementing management strategies to restore thermal conditions.
Riverine thermal regimes
- 1. Riverine Thermal Regimes An Integral Component of Environmental Flows Julian D. Olden School of Aquatic and Fishery Sciences University of Washington
- 2. Ecologically Sustainable Water Management Meeting the fresh water demands of a growing human population while ensuring ecosystem integrity has emerged as one of the world’s primary resource issues. Shifting focus from humans as exploiters of the environment to a world where riverine systems are legitimate “users” of fresh water.
- 3. The Science of Environmental Flows Scientists are becoming increasingly engaged in the development of environmental flow recommendations. An environmental flow is can be defined as the quantity, quality and timing of water flows required to maintain the components, functions, processes and resilience of aquatic systems which provide goods and services that are valued by society. Efforts in environmental flow research have thus far have focused primarily on water quantity, whereas issues involving water quality, such as water temperature, have received little consideration.
- 4. Why has temperature been overlooked in environmental flow science? Society is largely unaware of the ecological importance of a river’s thermal regime for freshwater biodiversity and ecosystem function. Scientists have their hands full with understanding how freshwater ecosystems respond to hydrologic modification and how best to use this information to inform the science of environmental flows. The impacts of river regulation on downstream thermal regimes are considered to be small compared to the potential for flow alteration.
- 5. Objectives The scientific community must broaden its perspective on ecologically sustainable water management to include aspects of the thermal regime in the science of environmental flows. 1. Review the concept of the natural thermal regime. 2. Illustrate how river regulation has modified thermal regimes and discuss the ecological impacts. 3. Challenge us to expand our perspective on environmental flows to include the management of thermal regimes for riverine ecosystem integrity.
- 6. Water Temperature Stream temperature is dependent on: Energy budget - the amount of heat energy added or loss to the channel. Thermal capacity - the volume of water to be heated or cooled.
- 7. The Natural Thermal Regime Stream temperatures show marked annual and diurnal fluctuations in response to seasonal and daily rhythms in the flux of heat energy gained and lost by a stream and the volume and source of runoff contributing to discharge. Bullpasture River, Virginia, USA
- 8. The natural thermal regime can be discomposed into its components of magnitude, frequency, duration, timing, and rate of change, and be summarized using statistics that describe the central tendency and variability in distributions of water temperatures. Mean annual T and CV Predictability/Constancy Magnitude Mean monthly T and CV Frequency 1-/3-/7-/30-day maxima T 1-/3-/7-/30-day minima T Descriptors of the Thermal Regime Duration High pulse count and duration Low pulse count and duration Timing Maximum and minimum T Rate of Change Rise and fall rates # reversals
- 9. Ecological Importance of Temperature Water temperature directly influences the metabolic rates, physiology, and life-history traits of aquatic species and helps determine rates of important ecological processes. Freshwater fishes and aquatic macroinvertebrates utilize a diverse array of thermal habitats to meet their specific temperature requirements for survival, growth and reproduction. Components of the thermal regime have different ecological significance.
- 10. Thermal Modification in Lotic Ecosystems Human activities may strongly alter spatial and temporal patterns in water temperatures by modifying the energy budget and thermal capacity of the river. Sources of “warmwater pollution” Forestry, agriculture, and urbanization • Industrial effluents • Irrigation diversions
- 11. Coldwater Pollution below Dams River regulation by dams can greatly modify water temperatures depending on their mode of operation and specific mechanism of water release. Large dams directly modify thermal regimes by releasing water that greatly differs in temperature from natural conditions (Storage Effect). By changing the volume of water, dams also influence temperature by affecting energy fluxes (Regulation Effect). Arthur Rylah Institute
- 12. Keepit Dam - Namoi River, Australia Annual maximum daily temperature was 5.0°C lower and occurred 3 weeks later compared to pre-dam conditions. Magnitude of thermal alteration decreased with distance downstream from dam. Preece and Jones (2002)
- 13. Blowering Dam – Tumut River, Australia Reductions in natural summer temperatures of 13.0°C to 16.0°C. Contributes to coldwater pollution for 80 river km below the dam and >200 km in the Murrumbidgee River. Lugg (1999), Preece (2004)
- 14. Vilui Dam - Lena River, Siberia Post-dam temperatures were 2-5°C higher in early- summer and 2-3°C lower in mid-summer. Lui et al. (2005)
- 15. Flaming Gorge Dam – Green River, USA Summer temperatures (June-Aug) were, on average, 12.0°C lower compared to pre-dam conditions. Annual maximum daily temperature was 14.0°C lower and its timing shifted from end-July to mid-December. Vinson (2001)
- 16. Gathright Dam – Jackson River, USA Summer temperatures (June-Aug) were, on average, 6.0°C lower compared to pre-dam conditions. Annual maximum daily temperature was 8.0°C lower. Olden et al. (in review)
- 17. Ecological Implications of Coldwater Pollution Cold water releases can have lethal and sub-lethal effects. Water temperatures below tolerance limits result in species extirpation, and sub-lethal impacts include a slowing of physiological processes such as reproduction and growth. Lower spring/summer water temperatures compromise the structure and life-history of stream macroinvertebrate assemblages, and decrease the survival of egg, larval and adult fishes. Delayed timing of peak spring temperatures disrupt critical cues for initiating fish spawning and insect reproduction and emergence.
- 18. Thermal regimes do not satisfy the spawning requirements of native fishes Pre-dam Post-dam Gathright Dam, Jackson River, USA
- 19. Five major challenges to developing comprehensive environmental flow assessments that incorporate the critical temperature requirements of riverine ecosystems.
- 20. CHALLENGE 1 Improve our understanding of spatiotemporal variability in riverine thermal regimes Previous attempts to explore similarities in thermal regimes among rivers and develop formal classification systems have been limited. Quantifying thermal variability in undeveloped rivers is essential for establishing regional benchmarks needed to successful incorporating water temperature into environmental flow management. The paucity of continuous water temperature data represents a significant information gap, requiring a greater dependence on statistical modeling.
- 21. CHALLENGE 2 Quantify the degree to which dams alter riverine thermal regimes In general, the scientific community has inadequately quantified the magnitude and geographic extent of dam- induced thermal alteration. Formal investigations of how dams are altering the various facets of the thermal regime are needed, including ecologically-relevant components of magnitude, frequency, duration, timing and rate of change in temperature events. This research is critical for mounting a convincing argument that temperature should play a significant role in the science and management of environmental flows.
- 22. Flaming Gorge Dam – Green River, USA Green River is the largest tributary of the Colorado River. Flaming Gorge dam was constructed in 1962 for hydroelectric power and flood control.
- 23. Degree of Thermal Alteration Daily water temperature records • Pre-dam: 1958-1962 • Post-dam: 1963-1977 Mean annual temperature decreased 2.5°C from 8.8°C to 6.3°C. Annual variability in water temperatures (CV) decreased from 89% to 34%. Thermal regimes were >2 times more predictable in post-dam years.
- 25. Olden and Naiman (in prep)
- 26. Why has temperature been overlooked in environmental flow science? Society is largely unaware of the ecological importance of a river’s thermal regime for freshwater biodiversity and ecosystem function. Scientists have their hands full with understanding how freshwater ecosystems respond to hydrologic modification and how best to use this information to inform the science of environmental flows. The effects of river regulation on downstream thermal regimes are considered to be smaller compared to the potential for flow alteration.
- 27. Thermal vs. Hydrologic Alteration ↑Thermal Alteration 1-/3-/7-/30-day minimum Dec (Winter) Jan (Winter) Feb (Winter) 90-day minimum Nov (Winter) Mar (Winter) ↑Hydrologic Alteration Date of maximum July - Oct (Summer) Date of minimum Rise and fall rates Lower pulse count Olden and Naiman (in prep)
- 28. CHALLENGE 2 … • Data limitation is a significant problem. • “Desk-top” assessments have been used to identify and rank large dams based on potential to cause coldwater pollution according to intake depth, discharge, storage, etc … Queensland: 18 dams (Brennan, in prep) NSW: 9 dams (Preece 2004) Victoria: 24 dams (Ryan et al. 2001) DIPNR (2004)
- 29. CHALLENGE 3 Quantify the ecological consequences of altered thermal regimes • Systematic assessments of the relationship between biological condition and the degree of thermal alteration are needed. Place the ecological impacts of thermal pollution in the context of broader ecological disturbances associated with dams.
- 30. CHALLENGE 4 Demonstrate the availability and success of thermal pollution remediation strategies Various mitigation measures are available: • Multi-level outlet structures • Artificial destratification (large propellers that pump cold bottom water toward the surface) • Trunnions (piping system that draw water from different levels in the water column) • Surface pumps (large propellers that pump warm surface water into existing outlets) • Draft tube mixers • Submerged curtains (large curtains extending upwards from the bottom of the reservoir forcing all the release water to originate from the surface) • Stilling basins
- 31. Thermal Restoration below Flaming Gorge Dam – Green River, USA Daily water temperature records Pre-dam: 1958-1962 Post-restoration: 1978-2006 Mean annual temperature decreased 0.3°C from 8.8°C to 8.5°C. Annual variability in water temperatures (CV) decreased from 89% to 42%. Thermal regimes were >2 times more predictable in post-dam years.
- 32. Post-dam (1963-1977) Post-restoration (1978-2006)
- 33. CHALLENGE 5 Develop a multi-faceted perspective on environmental flows Understand the relationships between flow alteration, thermal alteration, (other dam-induced drivers of environmental change), and the integrity of riverine ecosystems. Develop conceptual models and assess different environmental flow strategies that include prescriptions for both flow and temperature regimes.
- 34. B A A. Thermal restoration below Flaming Gorge Dam (Vinson 2001) B. Flow restoration below Clanwilliam Dam (King et al. 1998)
- 36. Take Home Message • Dams can substantially modify riverine thermal regimes, which can result in significant ecological impacts. • The degree of thermal alteration below dams may greatly exceed the level of flow alteration. • The benefits of environment flows may not be fully realized unless critical aspects of the thermal regime are also considered. • Incorporating aspects of water quality into environmental flow science and management represents a necessary step forward in ecologically sustainable water management.