Summary
1. The thermal regime of rivers plays an important role in the overall health of aquatic ecosystems, including water quality issues and the distribution of aquatic species within the river environment. Consequently, for conducting environmental impact assessments as well as for effective fisheries management, it is important to understand the thermal behaviour of rivers and related heat exchange processes.

2. This study reviews the different river thermal processes responsible for water temperature variability on both the temporal (e.g. diel, daily, seasonal) and spatial scales, as well as providing information related to different water temperature models currently found in the literature.

3. Water temperature models are generally classified into three groups: regression, stochastic and deterministic models. Deterministic models employ an energy budget approach to predict river water temperature, whereas regression and stochastic models generally rely on air to water temperature relationships.

4. Water temperature variability can occur naturally or as a result of anthropogenic perturbations, such as thermal pollution, deforestation, flow modification and climate change. Literature information is provided on the thermal regime of rivers in relation to anthropogenic impacts and such information will contribute to the better protection of fish habitat and more efficient fisheries management.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2427.2006.01597.x

The thermal regime of rivers : a review

Fresh waters are particularly vulnerable to climate change because (i) many species within these fragmented habitats have limited abilities to disperse as the environment changes; (ii) water temperature and availability are climate-dependent; and (iii) many systems are already exposed to numerous anthropogenic stressors. Most climate change studies to date have focused on individuals or species populations, rather than the higher levels of organization (i.e. communities, food webs, ecosystems). We propose that an understanding of the connections between these different levels, which are all ultimately based on individuals, can help to develop a more coherent theoretical framework based on metabolic scaling, foraging theory and ecological stoichiometry, to predict the ecological consequences of climate change. For instance, individual basal metabolic rate scales with body size (which also constrains food web structure and dynamics) and temperature (which determines many ecosystem processes and key aspects of foraging behaviour). In addition, increasing atmospheric CO2 is predicted to alter molar CNP ratios of detrital inputs, which could lead to profound shifts in the stoichiometry of elemental fluxes between consumers and resources at the base of the food web. The different components of climate change (e.g. temperature, hydrology and atmospheric composition) not only affect multiple levels of biological organization, but they may also interact with the many other stressors to which fresh waters are exposed, and future research needs to address these potentially important synergies.

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Climate change and freshwater ecosystems: impacts across multiple levels of organization

SUMMARY 1. Despite escalating conflict over fresh water, recent years have witnessed a growing realisation that human society must modify its behaviour to ensure long-term ecological vitality of riverine ecosystems. In response, ecologists have been increasingly asked to guide instream flow management by providing ‘environmental flow’ prescriptions for sustaining the ecological integrity of riverine systems. 2. Environmental flows are typically discussed in the context of water releases from dams and water allocation for extraction (such as for urban use or irrigation), where there is general agreement that rivers need to exhibit some resemblance of natural flow variability necessary to support a functioning ecosystem. Although productive dialogue continues on how best to define environmental flows, these discussions have been focused primarily on water quantity without explicit consideration of many components of water quality, including water temperature – a fundamental ecological variable. 3. Many human activities on the landscape have modified riverine thermal regimes. In particular, many dams have modified thermal regimes by selectively releasing hypolimnetic (cold) or epilimnetic (warm) water from thermally stratified reservoirs to the detriment of entire assemblages of native organisms. Despite the global scope of thermal alteration by dams, the prevention or mitigation of thermal degradation has not entered the conversation when environmental flows are discussed. 4. Here, we propose that a river’s thermal regime is a key, yet poorly acknowledged, component of environmental flows. This study explores the concept of the natural thermal regime, reviews how dam operations modify thermal regimes, and discusses the ecological implications of thermal alteration for freshwater ecosystems. We identify five major challenges for incorporating water temperatures into environmental flow assessments, and describe future research opportunities and some alternative approaches for confronting those challenges. 5. We encourage ecologists and water managers to broaden their perspective on environmental flows to include both water quantity and quality with respect to restoring natural thermal regimes. We suggest that scientific research should focus on the comprehensive characterisation of seasonality and variability in stream temperatures, quantification of the temporal and spatial impacts of dam operations on thermal regimes and clearer elucidation of the relative roles of altered flow and temperature in shaping ecological patterns and processes in riverine ecosystems. Future investigations should also concentrate on using this acquired knowledge to identify the ‘manageable’ components of the thermal regime, and develop optimisation models that evaluate management

http://depts.washington.edu/oldenlab/wordpress/wp-content/uploads/2013/03/FreshwaterBiology_2010a.pdf

Incorporating thermal regimes into environmental flows assessments: modifying dam operations to restore freshwater ecosystem integrity

Research on stream and river temperatures is reviewed with particular attention being given to advances in understanding gained since 1990 and on investigations of fundamental controls on thermal behaviour, thermal heterogeneity at different spatial scales, the influence of human impacts and the nature of past and future trends. Copyright  2008 John Wiley & Sons, Ltd.

Recent advances in stream and river temperature research

Climate change will affect hydrologic and thermal regimes of rivers, having a direct impact on freshwater ecosystems and human water use. Here we assess the impact of climate change on global river flows and river water temperatures, and identify regions that might become more critical for freshwater ecosystems and water use sectors. We used a global physically based hydrological-water temperature modelling framework forced with an ensemble of bias-corrected general circulation model (GCM) output for both the SRES A2 and B1 emissions scenario. This resulted in global projections of daily river discharge and water temperature under future climate. Our results show an increase in the seasonality of river discharge (both increase in high flow and decrease in low flow) for about one-third of the global land surface area for 2071–2100 relative to 1971–2000. Global mean and high (95th percentile) river water temperatures are projected to increase on average by 0.8–1.6 (1.0–2.2) °C for the SRES B1–A2 scenario for 2071–2100 relative to 1971–2000. The largest water temperature increases are projected for the United States, Europe, eastern China, and parts of southern Africa and Australia. In these regions, the sensitivities are exacerbated by projected decreases in low flows (resulting in a reduced thermal capacity). For strongly seasonal rivers with highest water temperatures during the low flow period, up to 26% of the increases in high (95th percentile) water temperature can be attributed indirectly to low flow changes, and the largest fraction is attributable directly to increased atmospheric energy input. A combination of large increases in river temperature and decreases in low flows are projected for the southeastern United States, Europe, eastern China, southern Africa and southern Australia. These regions could potentially be affected by increased deterioration of water quality and freshwater habitats, and reduced water available for human uses such as thermoelectric power and drinking water production.

/pdf/global-river-discharge-and-water-temperature-under-climate-17lqqw41ei.pdf

Global river discharge and water temperature under climate change

River temperature is an important physical characteristic of water quality, and long-term monitoring in Austria has provided a unique perspective on river temperature changes during the 20th century. Significant rises in annual mean values were apparent and broadly followed changes in air temperature. However, especially in relation to seasonal mean values, trends were influenced by catchment characteristics and contrasts were evident between sites located in a headwater tributary, at the outlet of a catchment with a sizeable lake area, and on the mainstream of the Danube. There is evidence that inter-annual variations in water temperature were influenced by the climate pattern of the North Atlantic Oscillation, especially in the winter months.

Long-term changes in river temperature and the influence of climatic and hydrological factors

Monthly mean data from a 90 year period relating to a small catchment (142.4 km2) in north-central Austria were used to provide a long-term perspective on the nature of the air–water temperature relationship. Annual mean values of air and water temperature were related in a relatively insensitive and scattered way (r2   95%, b > 0.65). A separate regression equation was needed to describe the behaviour of monthly mean water temperatures as the air temperatures fell below freezing. Analysis of air–water temperature regressions for individual months revealed a series of relations which were generally more scattered and less, but variously, sensitive than the ensemble relationship of monthly mean values. Monthly mean water temperatures could be predicted from the ensemble air–water temperature relationship and from the relations for individual months with root mean square errors of > 1.0 and < 0.8°C, respectively. Segmentation of air–water temperature regressions according to air temperatures above and below freezing did not significantly improve water temperature prediction. Hysteresis in, and the relatively low slope of, the air–water temperature relationships in the study catchment reflected the importance of snowmelt in the flow regime. © 1997 by John Wiley & Sons Ltd.

Long‐term perspective on the nature of the air–water temperature relationship: a case study

Monthly mean data collected for a 90 year period (1901–1990) have been used to study long term water temperature trends at 10 river sites in Austria. Significant increases in monthly mean values were evident over the study period for seven river stations, but the magnitude of the rise varied from 0.47 to 1.26°C and its pattern was not uniform between catchments. River temperature trends for individual months also ranged from increases of >2.5°C, through no significant change, to decreases of >1.0°C. Rises in water temperature during the twentieth century appear to reflect, at least in part, the increasing impact of human activity on Austrian rivers. Progressive regulation of main rivers, such as the Danube and Inn, has resulted in strong increases in mean water temperatures during the low flow period of autumn but not in the summer months when high flows generally occur. Different trends were evident for sites closer to glacial runoff sources, for catchments containing a large proportion of lakes...

Franz Nobilis

Papers

Recent advances in stream and river temperature research

Long-term changes in river temperature and the influence of climatic and hydrological factors

Long‐term perspective on the nature of the air–water temperature relationship: a case study

Long term water temperature trends in Austrian rivers

River and stream temperature: dynamics, processes, models and implications