Incorporating thermal regimes into environmental flows

assessments: modifying dam operations to restore

freshwater ecosystem integrity

JULIAN D. OLDEN AND ROBERT J. NAIMAN

School of Aquatic and Fishery Sciences, University of Washington, Seattle, WA, U.S.A.

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

Correspondence: Julian Olden, School of Aquatic and Fishery Sciences, Box 355020, University of Washington, Seattle, WA 98195,

U.S.A. E-mail: olden@u.washington.edu

Freshwater Biology (2010) 55, 86–107 doi:10.1111/j.1365-2427.2009.02179.x

86 2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd
trade-offs and provide a range of optimal environmental flows that meet both ecosystem

and human needs for fresh water.

Keywords: coldwater pollution, hypolimnetic release, water quality, water temperature

Introduction

The availability of fresh water to simultaneously meet

the demands of a growing human population and

ensure freshwater ecosystem integrity has emerged as

one of the world’s primary resource issues (Alcamo

et al., 2008). As world water demand has more than

tripled over the last half-century, signs of water

scarcity have now become commonplace. Given the

realities of human dependency on freshwater ecosystems in the future, ecological thought has evolved

from a focus on humans as exploiters of riverine

ecosystem services to a world where humans and

freshwater systems must coexist to ensure long-term

ecological sustainability (Palmer et al., 2004). This

represents a formidable challenge in the coming

decades, which will only get more difficult with

projected growth in the human population, intensified

land use and a changing climate (Poff et al., 2003;

Palmer et al., 2008).

Achieving compatibility between human and natural ecosystem needs is the ultimate challenge of

ecologically sustainable water management (sensu

Richter et al., 2003), in which scientists are becoming

increasingly engaged. One of the most promising

approaches to integrating human uses into the larger

scope of ecological sustainability is the concept of

environmental flows, or the provision of water within

rivers to conserve freshwater biodiversity while

maintaining the water needs of human society (Acreman & Dunbar, 2004). Environmental flows are

typically discussed in the context of water releases

from dams and catchment abstraction management

(i.e. water allocation strategies for urban use or

irrigation), where there is general agreement that they

need to exhibit patterns of natural variability to

support a functioning riverine ecosystem (Dunbar,

Acreman & Kirk, 2004; Richter & Thomas, 2007).

Recent dialogue has emerged among scientists on

how best to define and prescribe environmental flows

in a managerial context (e.g. Acreman & Dunbar,

2004; Arthington et al., 2006; Richter et al., 2006; Poff

et al., 2010), yet these discussions have focused

primarily on water quantity without the explicit

consideration of water quality, such as water temperature, pollutants, nutrients, organic matter and sediments and dissolved oxygen.

Hydrologic alteration is a major consequence of

river regulation associated with significant impacts on

aquatic biodiversity (Bunn & Arthington, 2002); however, dams and diversions have also greatly modified

riverine thermal regimes depending on their mode of

operation and specific mechanism and depth of water

release (Ward, 1985). For example, a substantial

number of large dams throughout the world have

intentionally managed thermal regimes by selectively

releasing cold water from deep reservoirs to establish

highly desirable fishing opportunities for trout,

salmon or walleye. Similarly, fluctuating releases by

hydroelectric dams from beneath the reservoir’s thermocline can result in highly variable and frequently

depressed summer water temperatures. Smaller dams

and diversions, in contrast, can cause increases in

downstream temperatures by releasing warm water

directly from the reservoir surface. Dam-induced

modifications to a river’s thermal regime (also termed

thermal pollution) can have both direct and indirect

consequences for freshwater ecosystems, yet it has

been relatively unappreciated in discussions of instream flow management.

While the ecological significance of water temperature in riverine ecosystems is widely acknowledged

(reviewed in Magnuson, Crowder & Medvick, 1979;

Poole & Berman, 2001; Caissie, 2006), the prevention

or mitigation of thermal pollution below dams has

received little attention when environmental flow

assessments are conducted. This is despite the highly

recognised impact of dams on thermal regimes and

the inherent relationship between discharge, temperature and a host of other water quality variables

(Nilsson & Reno ̈fa ̈lt, 2008). Clearly, both discharge

and temperature must be simultaneously considered

for the successful implementation of environmental

flow management below dams. We perceive this as a

Incorporating thermal regimes in environmental flows assessments 87

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
major gap in environmental flow assessments that

needs to be both elaborated upon and addressed in

the future. Our article aims to narrow this knowledge gap by exploring the concept of the natural

thermal regime, reviewing how river regulation by

dams modify thermal regimes, and discussing the

ecological implications of thermal alteration for

freshwater ecosystems. We focus on the thermal

effects of dams, although we readily acknowledge

that many other human activities such as the direct

extraction of water for consumption, sanitation and

irrigation influence riverine thermal regimes and are

highly relevant in environmental flow assessments.

Next, we discuss five major challenges for incorporating water temperatures into environmental flow

assessments and highlight research opportunities

and some alternative approaches for confronting

those challenges. The perspectives developed in this

study are that by viewing environmental flows in a

holistic manner, in which both water quantity and

quality are considered in flow recommendations, we

will increase the chances of long-term success in

achieving ecologically sustainable water management.

The natural thermal regime and its importance

for riverine ecosystems

Stream temperature depends on the amount of heat

energy added or lost to the channel (i.e. energy

budget) and the volume of water to be heated or

cooled (i.e. thermal capacity) (Poole & Berman, 2001;

Caissie, 2006). Heat energy is exchanged both at the

air⁄surface water interface and at the streambed⁄water

interface through a number of physical processes,

which are mediated by a multitude of external drivers

that control heat and water delivery to the stream

(Fig. 1). Heat flux at the air⁄surface water interface

occurs primarily through solar or short-wave radiation, long-wave radiation, evaporative processes and

convective transfer resulting from temperature differences between the stream and the atmosphere. Heat

exchange at the streambed⁄water interface is mainly a

function of geothermal heating through conduction

and advective heat transfer through groundwater

inputs and hyporheic exchange. The rates of these

processes are controlled by a number of factors

related to atmospheric conditions, topography,

streambed and stream discharge (Webb, 1996; Caissie,

2006).

Water temperatures show marked annual and

diurnal fluctuations in response to seasonal and daily

rhythms in the amount and type of heat energy

gained and lost by a stream and the volume and

source of runoff contributing to discharge (Ward,

1985; Webb, 1996). A stream’s thermal regime

describes the distribution of the magnitude of water

temperatures, the frequency with which a given

temperature occurs, the time of the day or year when

a certain temperature occurs, and the duration of time

Fig. 1 Heat exchange processes responsible for variability in water temperatures

(bold font) and the physical drivers that

control the rate of heat and water delivery

to stream and river ecosystems (italic

font).

88 J. D. Olden and R. J. Naiman

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
for which a stream is above or below a given

temperature. We propose that the natural thermal

regime can be discomposed into its components of

magnitude, frequency, duration, timing and rate of

change. Therefore, much like the natural flow regime

(Poff et al., 1997) and the quantification of its characteristic properties (Olden & Poff, 2003), thermal

regimes can be summarised using statistics that

describe the central tendency and variability in

distributions of temperature conditions (Fig. 2a). For

example, Jackson, Gibbins & Soulsby (2007) calculated

a suite of metrics describing what they deemed as

critical attributes of the thermal regime for invertebrate communities in the Lyon River (Scotland). Such

analyses could also include the characterisation of

thermal regimes at multiple temporal scales (Steel &

Lange, 2007). In addition, the physiological tolerances

of freshwater species to particular temperatures (see

below for further discussion) and the influence of

temperature on other aspects of water quality, such as

solute and pollutant fluxes, nutrient concentrations,

organic matter and sediments and dissolved oxygen

(Webb, 1996; Caissie, 2006), allow for the quantification of more targeted metrics describing thermal

characteristics of streams and rivers. For example,

development schedules for freshwater fish and insects

respond to the summation of thermal units (i.e. the

accumulation of daily temperatures above some

threshold) as well as absolute temperatures, and fish

species have both chronic and acute temperature

(a)

(b)

Fig. 2 (a) A stream’s thermal regime describes the magnitude, frequency, duration, timing and variability in water

temperatures at different spatial and

temporal scales. Shown are a number of

quantitative metrics that describe different

components of the thermal regime,

including high thermal events (HTE) and

low thermal events (LTE) based on a

defined upper and lower threshold,

respectively. The threshold could be

ecologically derived (e.g. upper and lower

lethal temperature, threshold temperature

for cueing spawning or positive growth)

or based on the statistical distribution of

annual temperatures (e.g. 90th and 10th

percentile). Other metrics not illustrated

here could be appropriate for describing

thermal regimes for various objectives.

(b) A stream’s natural thermal regime

influences freshwater biodiversity via

multiple mechanisms that operate at

different spatial and temporal scales.

Shown are a number of examples in which

temperature influences the bioenergetic

and life-histories of fish (italic font),

insects (normal font) and riparian plants

(underlined font). Overall, the native

biotas of riverine ecosystems have

evolved in response to the natural thermal

regime.

Incorporating thermal regimes in environmental flows assessments 89

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
thresholds for survival, growth and reproduction

(Vannote & Sweeney, 1980; Coutant, 1987). This

provides the opportunity to derive synthetic metrics

that are ecologically relevant for the species or

community of interest.

The ecological integrity of lotic ecosystems depends

on the natural dynamics of the thermal regime. Water

temperature directly influences the metabolic rates,

physiology and life-history traits of aquatic species

and helps determine rates of important ecological

processes such as nutrient cycling and productivity,

and indirectly mediates biotic interactions (Magnuson

et al., 1979; Petts, 1986; Poole & Berman, 2001; Caissie,

2006; Webb et al., 2008). As ectotherms, freshwater

fishes and insects use a diverse array of thermal

habitats to meet their specific temperature requirements for survival, growth and reproduction (Coutant, 1987; Vinson & Hawkins, 1998). Consequently,

specific components of the thermal regime have

specific ecological relevance for freshwater organisms

throughout their life history. We highlight a few

examples in Fig. 2b.

Water temperatures directly influence growth rates

of aquatic organisms and interacts with the lower and

upper incipient lethal temperatures to shape species

distributions. The availability of thermal heterogeneity also provides ecothermic organisms the opportunity to select habitats that optimise their energy

intake relative to physiological costs and thus affecting growth and survival. For example, species of

salmon (Oncorhynchus spp.) thermoregulate behaviourally by occupying cooler areas, such as seeps and

confluences with cold streams, when surrounding

temperatures exceed their upper tolerances (e.g.

Berman & Quinn, 1991). Similarly, temperate fishes

actively seek warm thermal refugia provided by

small streams and springs to escape cold waters and

take advantage of warmer conditions needed for

optimal feeding efficiency and growth (e.g. Peterson

& Rabeni, 1996). Development and life-history events

of locally adapted populations of aquatic insects and

fishes are closely timed to prevailing thermal regimes

(Vannote & Sweeney, 1980; Vinson & Hawkins, 1998;

Huryn & Wallace, 2000). For example, the natural

temperature regime of a river provides thermal cues

that stimulate fish migration, spawning and egg

hatching, and directly influences egg survivorship

and developmental time (Wootton, 1990; Fig. 2b).

Changes in temperature can also influence the

type and prevalence of diseases affecting fish. Lastly,

the accumulation of daily maximum temperatures

above a critical threshold has been shown to be a

fundamental parameter in shaping the distribution

and condition of many aquatic species (Armour,

1991).

Dam-induced modifications to riverine thermal

regimes

Spatial and temporal patterns in stream temperatures

are influenced by modifications to the energy budget

and⁄or the thermal capacity of the fluvial ecosystem

(Poole & Berman, 2001). Any natural process or

human activity that alters the external drivers of heat

load or stream discharge in a system will influence

spatiotemporal patterns of water temperature

(Fig. 1). For example, the discharge of heated industrial effluents, such as cooling water from power

generating stations, can result in unnaturally elevated

water temperatures (Langford, 1990). A reduction in

stream discharge resulting from water extraction for

human consumption or irrigation practices can also

directly affect water temperatures by decreasing

thermal capacity and thus increasing the likelihood

of high temperature events (Sinokrot & Gulliver,

2000). In an indirect manner, stream temperatures

may be modified through changes in catchment land

use and channelisation, both of which alter the

energy budget and thermal capacity of the water

course (Moore, Spittlehouse & Story, 2005; Nelson &

Palmer, 2007).

Here, we focus on dams and diversions that have

both direct and indirect effects on riverine thermal

regimes. In general, the extent to which a dam affects

downstream thermal regimes depends on their mode

of operation and specific mechanism of water release.

Dams directly modify a river’s thermal regimes by

releasing water that differs greatly in temperature to

that occurring naturally in the river. The magnitude of

thermal alteration depends largely on stratification

behaviour of the reservoir (i.e. depth profile of the

thermocline), and the depth at which water is released

from the dam. Dams also modify water temperatures

indirectly by influencing processes controlling the

delivery, distribution and retention of heat within the

river channel. Changes to discharge and the volume

of water in a river, for example, affect the rate at

which water heats and cools in response to natural

90 J. D. Olden and R. J. Naiman

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
diurnal heat exchange both at the air⁄surface water

and streambed⁄water interface (Petts, 1986; Palmer &

O’Keeffe, 1989; Poole & Berman, 2001).

Many large dams throughout the world cause

downstream modifications to riverine thermal regimes by altering flow regimes and releasing cold

water from below the thermocline of the reservoir (i.e.

the hypolimnetic layer). Coldwater thermal regimes

are established by releasing water from a single deep

portal (often associated with hydroelectric generation)

or selectively withdrawing water from different reservoir depths. In many instances, hypolimnetic

releases of cold water provide unique and highly

desirable fishing opportunities for trout or salmon in

geographic regions that could otherwise not support

coldwater species (e.g. Brooker, 1981; Horne, Rutherford & Wehrly, 2004; Krause, Newcomb & Orth, 2005).

Although dam operations provide optimal coldwater

conditions for tailwater fisheries, this is often at the

detriment of entire warmwater assemblages of native

fishes and other aquatic organisms (see discussion

below). Consequently, many ecologists consider this a

form of thermal depression or coldwater pollution

because water temperatures are typically much lower

during the spring and summer months compared to

free-flowing rivers (although during the winter

months, dam operations typically elevate water temperatures in relation to natural conditions). Many

other dams (typically smaller in size and located in

cooler summer climates) release water from above the

thermocline of the reservoir (i.e. the epilimnetic layer)

resulting in elevated spring–summer water temperatures (e.g. Lessard & Hayes, 2003).

The thermal impacts associated with dam operations in riverine ecosystems are well recognised

(Ward & Stanford, 1982; Ward, 1985; Petts, 1986;

Webb & Walling, 1996); although they are examined

much less often than river hydrology. A reservoir acts

as a buffer that reduces both the annual amplitude of,

and daily fluctuations in, downstream temperatures

because the reservoir’s mass warms and cools more

slowly than the free-flowing river. Thermal effects of

river regulation by dams vary depending on the

landscape position of the dam, mode of dam operation, release depth and environmental and geomorphologic setting. Despite these many factors, an

examination of rivers regulated by large, hypolimnetic-release dams from different regions of the world

reveals a relatively consistent pattern (Fig. 3). In

general terms, reservoirs moderate downstream thermal regimes where temperatures are lower in the

spring and summer months, higher in the winter

months, fluctuate less seasonally, and exhibit delayed

timing of maxima compared to natural conditions.

Finer-scale investigations also show that water temperatures downstream from dams often have muted

diurnal variation because of the stable temperatures of

hypolimnetic waters (Lowney, 2000; Steel & Lange,

2007).

There are many similar examples to that presented

in Fig. 3 that could be cited (see references listed

above). In Australia, river regulation by hypolimnetic

dams has caused the annual thermal maxima of

numerous systems to be both reduced and displaced

in time; for example, downstream of Burrendong Dam

on the Macquarie River (8–12 C depression and 1–

3 month delay) and downstream of Dartmouth Dam

on the Mitta Mitta River (8–10 C depression) (reviewed in Lugg, 1999; Ryan et al., 2001; Preece, 2004).

Jackson et al. (2007) found that summer water temperatures in the regulated Lyon River were 5–6 C

cooler than the adjacent unregulated Lochay River,

Scotland. Similarly, Angilletta et al. (2008) reported a

decrease in summer water temperature from 17.5 to

14.5 C after the construction of Hills Creek Dam on

the Willamette River, Oregon (U.S.A.). These examples highlight a general pattern toward summer

cooling and winter warming resulting from hypolimnetic release operation below large dams. Not surprisingly, a contrasting pattern emerges when we

examine epilimnetic-release dams. Lessard & Hayes

(2003) examined 10 small dams on Michigan streams

(U.S.A.) and found that summer temperature below

dams increased an average of 2.7 C, ranging from a

1.0 C cooling to a 5.5 C warming.

Scientists have also provided a wealth of information concerning the extent to which the thermal effects

persist downstream beyond the immediate vicinity of

the dam. Thermal impacts can encompass relatively

short or extremely long distances below their respective dams depending on heat exchange with the

atmosphere, hydrologic inputs from tributaries and

groundwater recharge, and dam discharge (Palmer &

O’Keeffe, 1989). In the Murray-Darling River Basin

(Australia), depressed summer temperatures extend

up to several hundred kilometres downstream of

dams on a number of major rivers, including Murrumbidgee, Marquarie, Mitta Mitta, Namoi and

Incorporating thermal regimes in environmental flows assessments 91

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
Murray (Ryan et al., 2001; Preece, 2004; Todd et al.,

2005). The thermal recovery of the River Svratka,

Czech Republic, required over 40 km (Ward, 1985),

and using rates of maximum summer warming,

Stevens, Shannon & Blinn (1997) estimated that a

mainstem distance of 930 km would be required for

water temperatures to fully recover (increase to predam conditions) below Glen Canyon Dam in the

Colorado River, U.S.A. (a distance that is currently

prevented by other downstream dams). In coolwater

systems affected by epilimnetic-release dams, streams

may not be able to shed added heat during the

summer and downstream water temperatures may

continue to warm due to normal stream processes.

Fraley (1979) found significant summer temperature

increases in the Madison River, Montana (U.S.A.) that

never returned to upstream temperatures even 56 km

downstream of a surface-release dam, although

Fig. 3 Annual thermographs from regulated rivers across the world illustrating summer cooling and winter warming caused by

hypolimnetic-release dams. Mean monthly temperatures are presented for the unregulated record (filled symbols and solid line)

and the regulated record (open symbols and dashed line). The unregulated record includes data recorded either in pre-dam years (i.e.

prior to construction), immediately upstream from the dam, or from an unregulated tributary, and the regulated record includes data

recorded in post-dam years. Details regarding the temperature data are as follows. Flaming Gorge Dam: unregulated (1958–62),

regulated (1963–77), data source (US Geological Survey); Kielder Dam: unregulated (1993–95), regulated (1993–95, River Rede),

data source (David Archer); Vilui Dam: unregulated (1950–65), regulated (1966–97), data source (Daqing Yang); Gathright Dam:

unregulated (1991–2006, upstream from dam), regulated (1991–2006, downstream from dam), data source (US Geological Survey);

Xinanjiang Dam: unregulated (1957–59), regulated (1960–68), data source (Zhong & Power, 1996); Keepit Dam: unregulated

(1976–2003, upstream from dam), regulated (1976–2003), data source (Australian Department of Natural Resources).

92 J. D. Olden and R. J. Naiman

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
diurnal temperature fluctuations did recover with

increasing distance.

Ecological consequences of modified thermal

regimes

Extensive dam construction throughout the world has

raised long overdue concerns regarding the potential

impacts of altered thermal regimes on biodiversity

and ecological processes. Empirical evidence suggests

that dam-induced thermal alteration by dams has

significant implications for stream productivity and

the reproduction, growth, distribution and assemblage structure of organisms (Haxton & Findlay,

2008). Despite this, our current level of understanding

continues to be much less than our knowledge of the

ecological implications of altered hydrology (Murchie

et al., 2008). Below we illustrate the ecological consequences of modified thermal regimes through a series

of case studies.

In Australia, the ecological consequences of coldwater pollution have been more widely recognised

compared to other countries. A number of empirical

and modelling studies have explored the relationship

between dam-induced thermal changes and fish populations in rivers. Cooling and delayed timing of

maximum temperatures in the Namoi River below

Keepit Dam had significant consequences for the

spawning success of several native fish species (Preece

& Jones, 2002). Based on the percentage of time in the

spawning period that the mean daily water temperature exceeded the temperature threshold for spawning,

spawning opportunities for silver perch (Bidyanus

bidyanus Mitchell, 1838) and golden perch (Macquaria

ambigua Richardson, 1845) were reduced to 25–70%

and 44–87% of pre-dam years, respectively. Using a

stochastic population model, Todd et al. (2005) explored the impact of altered thermal regimes on the

population viability of Murray cod (Maccullochella peelii

peelii Mitchell, 1838) in the Mitta Mitta River downstream of the fourth largest dam in Australia, Dartmouth Dam. Model predictions showed that cold

water releases significantly threaten the post-spawning

survival of Murray cod by reducing the average

minimum female population size by 76% and increasing population variability by 137%.

In the Qiantang River Basin of China, the construction of Xinanjiang Dam caused mean temperatures to

decrease from 19.0 to 13.5 C, and the annual sum of

degree days >15 C (considered the positive-growth

threshold temperature for warmwater fish species)

dropped 37% (Zhong & Power, 1996). As a result, the

majority of warmwater fishes were extirpated from

large sections of river below the dam. In the Hanjiang

River below the Danjiangkou Dam, China, the postponed timing of spring–summer warming caused by

dam operations resulted in fish spawning times that

were delayed by 20–30 days. Late hatching and lower

water temperatures reduced the first year growth

compared to an unregulated river (Zhou, Liang &

Huang, 1980). Similarly, significant declines of native

fishes in the Colorado River Basin (U.S.A.) have been

attributed, in part, to the depression of spring–

summer tailwater temperatures caused by hypolimnial-release dams. For example, the operation of

Flaming Gorge Dam on the Green River, lowered

spring–summer tailwater temperatures to nearly 6 C

from a previous range of 7–21 C, which contributed

to the extirpation of several native species including

the endangered Colorado pikeminnow (Ptychicheilus

lucius Girard, 1856), humpback chub (Gila cypha

Miller, 1946) and bonytail chub (G. elegans Baird &

Girard, 1853) (Clarkson & Childs, 2000). Similarly,

Olden (2004) found that depressed summer temperatures below Gathright Dam on the Jackson River,

Virginia (U.S.A.), resulted in complete species turnover from a warmwater fish assemblage to a river

dominated by cool- and coldwater fish species.

Modified temperature regimes also strongly influence invertebrate communities by eliminating key

developmental cues and influencing the rate of egg

development and juvenile growth. Consequently,

changes in temperature can cause the asynchrony of

the life cycle to seasonal patterns in resource and mate

availability (Vannote & Sweeney, 1980). Individual

species responses can generate significant changes in

assemblage structure by shifting the composition from

warm stenothermic species to cold-tolerant eurytherms (Ward, 1974). For example, when a hypolimnetic dam was constructed on the Saskatchewan River

(Canada), the cue to end egg diapause was eliminated

because winter temperatures were maintained near

4 C and the insect biota no longer experienced a

prolonged period near freezing followed by a rapid

temperature increase (Lehmkuhl, 1974). This resulted

in the complete loss of an insect fauna, switching from

a river containing 12 orders, 30 families and 75 species

to one that comprised only the midge family ChiroIncorporating thermal regimes in environmental flows assessments 93

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
nomidae. Another potential consequence of warmer

winter temperatures is an increase in the growth of

aquatic insects, resulting in winter rather than spring

or early summer emergence (Ward & Stanford, 1982).

Emergence during the winter is either lethal or

impedes mating of aquatic insects. In another example, Stevens et al. (1997) found that the macroinvertebrate fauna of the Colorado River (U.S.A.) below Glen

Canyon Dam was highly depauperate compared with

other unregulated rivers of the basin. They collected

virtually no species from the Ephemeroptera (mayfly),

Plecoptera (stonefly) or Trichoptera (caddisfly) families in the mainstream in 1991, suggesting that coldstenothermic releases may not permit these taxa to

complete their life cycle. Voelz & Ward (1991)

observed increasing insect species richness with distance downstream from Granby reservoir dam (Colorado River, U.S.A.) in response to a gradual recovery

(increasing) in maximum summer stream temperatures. Additional examples of the effects of hypolimnetic release on macroinvertebrate communities are

discussed by Haxton & Findlay (2008).

Dam-induced changes in thermal regimes may also

have long-term evolutionary consequences for riverine biota by inducing a mismatch between a species’

life-history and other critical environmental conditions. For example, Angilletta et al. (2008) hypothesised that warmer temperatures during the autumn

and winter below Lost Creek Dam (Rogue River,

U.S.A.) may indirectly influence the fitness of Chinook

salmon (Oncorhynchus tshawytscha Walbaum, 1792) by

accelerating the development of embryos, leading to

earlier timing of emergence. Shifts to earlier emergence could lead to mortality from high flow events,

elevated predation or insufficient resources. Using an

age-based population model the authors predicted a

decrease in mean fitness of Chinook salmon after dam

construction. Although the likelihood for these impacts is unknown (i.e. temperature changes may also

result in strong compensatory strategies, such as

delayed spawning by adults or slowed development

by embryos), the potential evolutionary consequences

of thermal alteration should not be overlooked.

Challenges and prospects for incorporating thermal criteria into environmental flow assessments

We propose that a river’s thermal regime is a key, yet

poorly acknowledged, component of environmental

flows. The efficacy of environmental flows to advance

ecologically sustainable water management is predicated upon a greater appreciation of the influence of

human activities on riverine thermal regimes and

other critical water quality variables, and requires

new research and tools that help define management

options that maintain key hydrologic and thermal

processes for the benefit of aquatic ecosystems. Below,

we discuss five overarching challenges for incorporating thermal criteria into environmental flow assessments, and lay out some alternative approaches for

confronting those challenges. Some challenges are

deceptively simple, while others will require new

avenues of research. In the sections that follow we use

the Flaming Gorge Dam – a 91 m dam constructed in

1962 on Green River, U.S.A. (the largest tributary of

the Colorado River) for hydroelectricity generation

and flow control – as a working example. Although

our discussion, in part, focuses on the role of river

regulation by dams and diversions, we believe that

many of the challenges are equally pertinent for our

ability to incorporate water temperature targets into

environmental flows standards for water abstraction

practices.

Challenge #1: Advance our understanding of

dam-induced impacts to riverine thermal regimes

Continuing research has emphasised the complexity

of thermal responses to dams as a result of processes

occurring within reservoirs and downstream. Significant inter-annual variation in the impact of dam

operations on downstream temperatures has been

illustrated and attributed to year-to-year changes in

climate conditions, reservoir operation and behaviour,

and tributary inflows (Webb & Walling, 1993, 1996;

Preece & Jones, 2002; Todd et al., 2005). We highlighted a number of river systems around the world

with qualitatively very different annual thermographs

between pre- and post-dam time periods (Fig. 3).

Similarly, the majority of published studies reporting

the thermal impacts effects of dams have focused

almost exclusively on simple comparisons of annual

or monthly temperatures before and after dam construction, often without supporting statistical analyses. While such comparisons are informative for

descriptive purposes, we argue that ecologists must

better formalise how dams are altering the various

components of the thermal regimes, including the

94 J. D. Olden and R. J. Naiman

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
magnitude, frequency, duration, timing and rate of

change in temperature events. Each component of the

thermal regime has distinct ecological relevance for

freshwater organisms, communities and riverine processes (Fig. 2b), which should ultimately guide the

appropriate characterisation of the thermal regime

and the selection of relevant descriptors (Fig. 2a).

Characterizing thermal regimes and assessing the

downstream impacts of dams will be challenging for

many of the same reasons that assessing hydrologic

variability and alteration continues to be difficult –

perhaps even more so. In the United States, water

temperature data are collected by a variety of federal,

state and local agencies, although the primary source

for long-term daily records is the U.S. Geological

Survey (also the main source for daily discharge data).

As of June 2008, the U.S. Geological Survey has

collected data from 24 856 gauges across the country,

yet only 7.4% of these gauges contain records of daily

water temperature for at least 1 year, and less than

1% of all gauges contain temperature data for at least

15 years. Moreover, most temperature records begin

after the construction of a dam or diversion. Given the

paucity and appropriateness of long-term water temperature data (a problem common to most other

regions of the world), investigators may be forced to

rely more heavily on temperature models based on

unregulated systems to predict riverine thermal conditions. These will include empirical models that use

statistical analyses (e.g. regression, neural networks)

to make temperature predictions from meteorological

data or catchment characteristics, physically based

modelling that involves solving the heat budget

equation and hydrodynamic models relating discharge to water temperatures (Gu, McCutcheon &

Chen, 1999; Caissie, 2006; Webb et al., 2008). Scientists

will also benefit greatly from recent technological

developments that have facilitated the measurement

and monitoring of water temperature using programmable (and remotely downloadable) digital loggers

and remotely sensed thermal infrared imagery (Webb

et al., 2008), in addition to the creation of central

depositories or information systems containing water

temperature data.

In some cases, long-term daily water temperatures

will be available (or modelled) to quantify the impact

of river regulation on thermal regimes. As an example, we quantitatively compared the thermal regimes

of the Green River (U.S.A.) before and after the

construction of Flaming Gorge Dam with respect to

metrics describing the magnitude, frequency, duration and timing of water temperatures. These thermal

metrics were selected a priori based on previous

empirical demonstrations of their ecological importance for species occurrence and community structure

in the Green River, or more broadly the Upper

Colorado River Basin (Clarkson & Childs, 2000;

Vinson, 2001). Flaming Gorge Dam uses a hypolimnetic release schedule that has increased predictability

and substantially reduced annual variability in water

temperatures (Fig. 4). Average late spring and summer temperatures (May–August) are 66% lower in

post-dam years decreasing from 17.2 to 5.7 C,

whereas winter temperatures (December–March)

showed a marked increase of 802% from 0.7 to

5.4 C (Fig. 4). Magnitude of 1-, 7- and 30-day minimum temperatures increased from 0–0.2 to 3.4–3.9 C

after dam construction, whereas the magnitude of

maximum temperatures for the same duration decreased 56% from 21.1–24.0 C (pre-dam) to 9.6–

10.0 C (post-dam) (Fig. 4). Dam operations have

resulted in significantly lower frequency and duration

of low and high temperature events (Fig. 4), and

shifted the timing of average minimum temperatures

forward 3 weeks (12th February–4th March) and

average maximum temperatures ahead over 15 weeks

from the summer (24th July) to the autumn (7th

November) (Fig. 4). We recommend that future

research should focus on the quantitative assessment

of thermal alteration associated with dam operations,

including the statistical analysis of pre- and post-dam

time periods to better elucidate how different components of the riverine thermal regime are modified

by dam operations and recovery downstream.

Without detailed and long-term temperature data

(either recorded or modelled), it is unlikely that

comprehensive evaluations of dam-induced thermal

alteration at the landscape scale will be possible.

However, in lieu of these data it is possible to

qualitatively forecast the thermal impacts of river

regulation according to dam⁄reservoir characteristics

(e.g. dam height, reservoir depth and bathymetry),

operation schedule (e.g. intake depth, discharge), and

geomorphic attributes of the receiving river system

(channel form, groundwater influences, position and

characteristics of tributaries). In Australia, the states of

Victoria, Queensland and New South Wales have

completed comprehensive reviews of large dams to

Incorporating thermal regimes in environmental flows assessments 95

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
identify and rank structures based on potential to

cause coldwater pollution. Ryan et al. (2001) ranked

dams in Victoria based on depth and frequency

(regular versus occasional) of release, and identified

24 top priority dams for which further assessment and

continuous temperature monitoring was recommended. The New South Wales review used intake

depth and summer discharge to develop a short list of

nine dams (out of 93 in total) predicted to cause severe

coldwater pollution (Preece, 2004). With the exception

of Australia, the potential for coldwater pollution

associated with dams has not been systematically

assessed. These so-called ‘desktop assessments’ may

be particularly valuable for identifying those rivers

where thermal pollution may be prevalent, thus

helping guide future temperature surveillance efforts

and prioritizing the possible retrofitting of current

gauges to record water temperatures. Moreover, in

combination with additional modelling, this information would enable the mapping of spatial patterns in

dam-induced thermal alteration across entire riverine

landscapes.

Challenge #2: Advance our understanding of the

ecological consequences of altered thermal regimes

Hydro-ecological studies are becoming increasingly

interdisciplinary, with research focusing on, among

other things, the relationships between water quantity

and quality, and their importance for riverine biota.

However, despite broad recognition of the importance

of flow and thermal regimes for river systems, there

are relatively few studies that have explicitly linked

dam-induced changes in both flow and thermal factors

to ecological structure and function. In a revealing

study, Murchie et al. (2008) conducted a systematic

review of the literature to identify studies that

examined the response of fish to modified flow

Fig. 4 Hypolimnetic release operations of

Flaming Gorge Dam have significantly

altered the thermal regime of the Green

River (U.S.A.). Comparisons of thermal

metrics computed for the pre-dam (1958–

62, circles) and post-dam records (1963–

77, squares) show considerable effects on

the thermal regime, including variability

and predictability of annual temperatures,

monthly temperatures, magnitude, frequency and duration and timing of thermal events. Low and high pulses (and

durations) are based on mean daily temperatures below the 25th percentile and

above the 75th percentile of unregulated

temperatures, respectively. Bars represent

±1 SD and ‘CV’ refers to the coefficient of

variation.

96 J. D. Olden and R. J. Naiman

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
regimes in regulated rivers. Of the 131 studies

identified in their review, they found that although

almost half of the studies included the collection of

water temperature, the majority of these studies (57%)

failed to examine the potential consequences of

thermal alteration. Moreover, an extremely small

number of studies included temperature data in

formal statistical analysis to explore the potential

individual and interactive effects of flow and thermal

modification on fish assemblages. We cannot emphasise enough the need for this research for supporting a

truly holistic approach to environmental flow assessments that incorporates both flow and temperature

targets.

The limited literature examining both thermal and

flow effects on freshwater organisms suggests that

both the direct and interactive influence of these

factors are critical. In the Jackson River (U.S.A.) below

Gathright Dam, Olden (2004) found that no one single

component of the flow or thermal regime explained

fish population- and community-level responses to

river regulation. Overall, species showed patterns of

numerical recovery below the dam that were largely

congruent with their ecological requirements; for

example, fluvial-specialist species responded positively to decreasing flow alteration and warmwater

species responded positively to decreasing thermal

alteration. Jackson et al. (2007) explored the relative

roles of discharge and temperature variation in

determining invertebrate community structure in a

regulated river in Scotland. The authors found that no

one flow or thermal metric exerted an overriding

influence on community structure suggesting that

whole regime changes, rather than alterations to one

particular aspect of the regime, were responsible for

the impoverishment of invertebrate communities

observed below the dam.

Although some additional examples do exist, in

general it remains extremely difficult to ascertain the

relative contributions of flow versus temperature (and

a number of other water quality variables influenced

by dams and human activities) to observed biotic

responses in riverine ecosystems (Bednarek & Hart,

2005). This is supported by Haxton & Findlay (2008)

who, after conducting a comprehensive review of the

literature, lamented the paucity of studies that quantified the effects of particular drivers of dam-induced

change on downstream species responses. We view

this as a significant challenge that currently precludes

our ability to formally incorporate thermal criteria

into environmental flow assessments. Confronting

this challenge may be facilitated by the simple fact

that levels of hydrologic and thermal alteration and

rates of downstream recovery vary substantially

within and among regulated rivers. By synthesizing

and collecting data along these spatial gradients of

alteration, greater insight into the relative roles of

hydrologic and thermal factors in shaping stream

communities will be possible.

Challenge #3: Demonstrate the availability and success

of temperature management strategies

A number of management options exist for mitigating

the thermal impacts of dams. The indirect management of water temperatures is possible by targeting

those drivers of thermal regimes that can be directly

manipulated, such as the appropriate management of

riparian zones and the maintenance of natural flow

variability (Poole & Berman, 2001; Fig. 1). The management of riverine landscapes for thermal integrity

will require a broad perspective that recognises the

heterogeneous nature in which the topology of the

drainage network controls the physical processes

shaping spatial and temporal variability in stream

temperatures. For example, the ecological importance

of tributaries for promoting physical heterogeneity is

well recognised (e.g. Rice, Ferguson & Hoey, 2006),

and we believe that understanding the dynamics of

confluence zones is an integral part of understanding

the role of tributaries in mediating the downstream

impacts of dams on thermal regimes (in addition to

hydrologic regimes and sediment processes). The

degree to which unregulated tributaries lessen the

thermal influences of dams will depend on the size of

the stream and its distance from the dam, and

characteristics of the tributary with respect to discharge, sediment and water temperature (Petts, 1986).

The influence of tributaries on mainstem thermal

and biological recovery has been illustrated in a

number of studies (e.g. Ward, 1985; Stevens et al.,

1997; Vinson, 2001). Preece & Jones (2002) showed that

the pattern of thermal recovery downstream from

Keepit Dam on the Namoi River (Australia) was

significantly influenced by the contribution of two

unregulated tributaries; the magnitude of this effect

was greater in the spring and early summer when

tributary discharge was 50% of that from Keepit

Incorporating thermal regimes in environmental flows assessments 97

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
Dam, compared to the summer when the relative

contribution of the tributaries was small (<10% of

Keepit Dam). Sato et al. (2005) found that the reproductive success of an important fisheries species,

Prochilodus argenteus Spix & Agassiz, 1829 in the cold

tailwaters below the Tres Marias Dam on the Sao

Francisco River, Brazil, was achievable only below the

confluence with a medium-sized, unregulated tributary. Vinson (2001) recorded greater insect richness

downstream from a tributary confluence on the Green

River below Flaming Gorge Dam, and Olden (2004)

found elevated richness of warmwater fish species in

the Jackson River below Gathright Dam in reaches

immediately downstream from incoming warmwater

tributaries. Understanding the role in which tributaries contribute to achieving prescribed thermal benchmarks in environmental flow assessments is a key

challenge that requires additional exploration.

In highly regulated systems, the management of

mainstem channel vegetation and tributary contributions may be insufficient to meet thermal regime

standards. In such cases, our ability to incorporate

temperature requirements in environmental flows

assessments will require suitable technologies for

directly modifying water temperature released from

dams. Current management options for mitigating

thermal impacts from dams divided into two general

categories: exploit the temperature stratification of

the reservoir by selective withdrawal of water of

the desired temperature, or artificially break up the

stratification prior to discharging water from the dam.

Sherman (2000) provided a review of seven general

strategies for mitigating thermal pollution; we briefly

summarise these below:

1 Multi-level intake structures make use of the

stratification within the reservoir by permitting water

with desirable thermal attributes to be withdrawn

from defined regions within the water column

(Fig. 5a). We discuss the details of this method in

subsequent paragraphs.

2 Trunnions (floating intakes) are a variation on the

theme of selective withdrawal that involves the intake

of epilimnetic and hypolimnetic water through two

pipes hinged at the dam wall (Fig. 5b).

3 A variety of destratification approaches exist that

induce mixing of the water column to raise the water

temperatures of the hypolimnion immediate above

the dam prior to release. Local destratification is

(a) (b)

(c) (d)

(e) (f)

Fig. 5 Current management options for

mitigating thermal impacts from dams

include: (a) selective withdrawal using

multi-level intake structures or (b) trunnions, (c) local destratification using aeration systems, (d) surface pumps or (e)

draft tubes and (f) submerged weirs or

curtains to redirect either hypolimnetic or

epilimnetic water. Figures were provided

by Brad Sherman (Australia’s Commonwealth Scientific and Industrial Research

Organisation: CSIRO).

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2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
accomplished by pushing cold water up to the surface

using aeration systems that introduce bubble plumes

or directly mixing using pumps (Fig. 5c). Bubble

plume destratification systems have been typically

applied in eutrophic reservoirs to improve overall

water quality, particularly to increase dissolved oxygen content and decrease the potential for cyanobacterial blooms, however this approach can also greatly

increase hypolimnetic temperature water adjacent to

the intake structure.

4 Similarly, surface pumps on floating platforms are

also used to locally destratify reservoirs by sending

surface water downwards into the withdrawal zone of

the intake structure (Fig. 5d).

5 Draft tubes can be anchored in the water column

to increase the distance travelled by the plume of

water from the propeller (Fig. 5e).

6 Submerged weirs or curtains suspended at various depths can be used to provide a barrier to the

passage of water and force warm or cold water above

or below the curtain, respectively (Fig. 5f). Modifying

the topography of the channel feeding into the dam

can also result in a similar effect.

7 Lastly, stilling basins (i.e. large shallow ponds)

have been used to delay the downstream release of

water so thermal equilibrium may be reached with the

atmosphere.

An insightful comparison of these technologies for

the Burrendong Dam (Australia) is provided by

Sherman (2000).

Selective withdrawal using a multi-level intake

structure is the most common and most effective

means of controlling the water temperature of dam

releases. A selective withdrawal system (also called a

temperature control device) can extract water from

selected depths of a thermally stratified reservoir to

produce a release with desired characteristics (Price &

Meyer, 1992). This technology provides the flexibility

to increase water temperatures by preferentially

selecting warm epilimnetic water from the surface,

or decrease water temperature by drawing cold

hypolimnetic water from below the thermocline. For

example, Flaming Gorge Dam (Green River, U.S.A.)

was installed with a multi-level intake structure in

1978 with the goal of increasing summer water

temperatures for native species in the trout-dominated tailwaters. The partial success of this thermal

restoration is evident by examining the annual thermographs below the dam during different time

periods (Fig. 6). After the installation of the intake

structure (1978–2007), average stream temperatures

during the summer months of May to August almost

doubled from a pre-installation (1963–77) temperature

of 5.7 to 11.0 C. Pre-dam temperatures (1958–62) in

the Green River during the summer averaged 17.2 C,

but restoring the tailwater to historical temperatures

was not the management goal (Fig. 6). This translates

to a dramatic decline in per cent thermal alteration

during the summer from )66% (pre-dam versus

pre-installation: Fig. 4b) to )36% (pre-dam versus

Fig. 6 Selective discharge below Flaming

Gorge Dam using a multi-level intake

structure has markedly decreased the

degree of thermal alteration in the Green

River (U.S.A.). Comparisons of monthly

water temperature during pre-dam (1958–

62, circles), post-dam (1963–77, squares)

and post-thermal restoration years (1978–

2007, triangles) shows significant increases in spring–summer temperatures

toward unregulated conditions. Notably,

current operations of the temperaturecontrol device have not decreased the

degree of thermal alteration during the

winter months.

Incorporating thermal regimes in environmental flows assessments 99

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
post-installation). Reasonable improvements were

also observed for the frequency and duration of

thermal events, and the timing of extreme temperatures. Minimum temperature after thermal restoration

occurred, on average, on 8th February (only 4 days

earlier than the pre-dam average date of 12th February), and mean day of year of maximum temperatures

recovered by moving 10 weeks earlier from 7th

November (pre-installment) to 18th August (postinstallation); now representing a 5-week delayed

timing compared to pre-dam years (24th July). The

biological implications of these thermal improvements are discussed later. With another management

goal in mind, Shasta Dam (Sacramento River, U.S.A.)

was retrofitted with a multi-level intake structure in

1997 to improve downstream temperatures for endangered coldwater salmonids. Dam management is

focused on releasing warmer surface waters in the

winter⁄spring and colder deep waters in the summer⁄autumn. These two examples illustrate the flexibility of multi-level intake structures for controlling

downstream temperatures for a defined ecological

goal.

Although most selective withdrawal intake structures are built during initial reservoir construction

(ironically, most were originally designed to support

tailwater trout fisheries), release structures can also be

successfully modified for selective withdrawal later

following initial construction (see both example

above). However, the capital costs of refitting dams

with a multi-level intake structure may be prohibitive.

For example, the installation cost for several large

dams in Australia was estimated to range between 3

and 30 million AUD (Sherman, 2000), and the actual

cost for retrofitting Shasta Dam was 80 million USD.

Estimated installation costs for modifying eight intake

portals of Glen Canyon Dam (Colorado River, U.S.A.)

is 15 million USD. Assuming operations occur during

the months of June through August and all releases

are surface withdrawals, the economic value of the

additional increases in head loss (resulting from the

modified intake) are estimated to be 228 000 USD -

year-1 (Vermeyen, 1999). Economic costs aside, there

are also a host of other operational, physical⁄chemical

and biological considerations that must be assessed

when decided whether to refit a dam with a temperature control device.

Despite the capital costs of installing multi-level

intakes, recent investigations suggest that these structures provide the most flexible means of modifying

downstream water temperatures, even at low to

medium release volumes (Sherman, 2000), and

may represent the best opportunities for ecological

restoration. Sherman et al. (2007) predicted that the

installation of a multi-level intake structure in Hume

Dam, Australia, would increase discharge temperatures by 4–6 C during the spring–early summer postspawning period for Murray cod. These temperature

increases were forecasted to increase minimum

female population abundance in the Murray River

by 30–300% depending on the assumed spawning

behaviour. Research continues on how epilimnetic

versus hypolimnetic withdrawal affects reservoir

water quality (e.g. vertical distribution of temperature, dissolved oxygen and nutrients) throughout the

year (Johnson et al., 2004), which will help determine

the optimal selective withdrawal release strategy.

Future research is required to compare the ability of

different mitigation strategies (Fig. 5) to meet required temperature standards defined in environmental flow prescriptions.

There is also opportunity to manage reservoir

operations to directly affect average daily downstream temperatures through adjustments in the

magnitude and timing of discharge releases. Polehn

& Kinsel (1997) examined this analytically and suggested that changes in flow may be used to adjust the

diurnal temperature cycle at specific locations downstream of a reservoir. However, Lowney (2000)

showed that an interesting pattern of ‘nodes’ and

‘antinodes’ in the extent of diel variation may develop

at different distances downstream from large dams if

any particular flow is sustained for more than a few

days. Consequently, the control of nodes and antinodes through flow change may require continuous

flow adjustment, possibly in conflict with project

objectives. In large systems where constant flow is

desired and a temperature control system may

already be in place, it is possible that a well designed

temperature release pattern could restore diurnal

variation to that expected under pre-dam conditions

(McMahon & Finlayson, 1995). However, meeting diel

thermal targets using modified release schedules will

be complicated in regulated systems because it will

not be possible to mimic the diel temperature variations of a natural system if patterns in discharge are

not also mimicked. Quite simply, variation in diel

temperatures decreases with increasing discharge

100 J. D. Olden and R. J. Naiman

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
due to the increase in thermal mass of the river.

Typically, flows released during the summer, either

for irrigation or recreation, exceed the natural flow for

a river. While it may be possible to modify the mean

temperature of the release using a particular mitigation technique, it is unlikely that the nature temperature range will also be restored. In general, the

feasibility of controlling river temperature through

streamflow management has been recognised for

some time, yet the quantitative temperature : flow

relationships required for implementing instream

flow requirements based on this principle have not

be successfully developed (Gu et al., 1999).

Challenge #4: Incorporate thermal-criteria into environmental flow assessments

Defining environmental flows with respect to either

flow or temperature in isolation is highly unlikely to

support ecologically sustainable water management.

Consequently, the benefits of environmental flows

will be realised only if riverine thermal regimes are

also considered. One example supporting this notion

is the study of King, Cambray & Impson (1998) who

examined patterns of temperature and discharge

resulting from an experimental dam release, and their

relative importance in triggering successful spawning

of a threatened large cyprinid endemic, the Clanwilliam yellowfish Barbus capensis Smith, 1841, downstream of Clanwilliam Dam (Olifants River, Africa).

A critical flow component for many rivers in the

winter-rainfall region of South Africa, including the

Olifants River, is the small pulses of higher flow that

occur in the dry season (called freshes). Experimental

freshes released from Clanwilliam Dam in the early

1990s were strongly correlated with the suspected

spawning success of B. capensis (Cambray, King &

Bruwer, 1997); however, in subsequent years freshes

delivered during the species’ breeding season failed to

induce spawning. King et al. (1998) found that differences in the temperatures of the water release were

the key factor related to spawning success. Specifically, warm epilimnetic freshes (19–21 C) triggered

fish spawning behaviour and the movement of individuals onto spawning beds, whereas cool hypolimnetic baseflows (16–18 C) released immediately after

the experimental freshets caused fish to abort spawning activities. Moreover, the occurrence of dead and

deformed young suggested that the cold water may

have had a detrimental effect on offspring of those

individuals who spawned during the warmwater

events. This research showed that freshets released

from Clamwilliam Dam at the appropriate time

should be able to induce spawning and support early

life stages of B. capensis only if water temperatures

at the spawning sites exceeded 19 C. Furthermore,

King et al. (1998) suggest that successful spawning

will lead to high recruitment only if water temperatures are maintained at these levels for an extended

period after spawning to provide for the development

of the embryos and larvae.

Similar examples exist in the lower Mississippi River

(U.S.A.) where research has shown that growth and

abundance of juvenile fishes are only linked to floodplain inundation when water temperatures are greater

than a particular threshold. Schramm & Eggleton (2006)

reported that the growth of catfishes (Ictaluridae spp.)

was significantly related to the extent of floodplain

inundation only when water temperature exceeded

15 C; a threshold temperature for active feeding and

growth by catfishes. Under the current hydrographic

conditions in the lower Mississippi River, the authors

report that the duration of floodplain inundation when

water temperature exceeds the threshold is only about

1 month year)1 on average. Such a brief period of time

is believed to be insufficient for floodplain-foraging

catfishes to achieve a detectable energetic benefit

(Schramm & Eggleton, 2006). These results are consistent with the ‘thermal coupling’ hypothesis offered by

Junk, Bayley & Sparks (1989) whereby the concordance

of both hydrologic and thermal cycles is required for

maximum ecological benefit.

Similarly, the advantages of thermal restoration

may be realised only if flow is also actively managed

to mimic natural regimes. As previously discussed,

the operation of Flaming Gorge Dam after installation

of a multi-level intake structure resulted in significant

warming of summer temperatures (Fig. 6) and increased the number of annual degree days from 2340

to 3200 (Vinson, 2001). While these improvements in

thermal regimes were predicted to increase invertebrate richness, Vinson (2001) found that the number of

taxa collected after partial thermal restoration was

similar to or lower than that observed before temperature manipulations. The lack of appreciable increase

in richness can be attributed, in part, to the remaining

differences in the thermal and hydrologic (and associated sedimentation processes) regimes of the Green

Incorporating thermal regimes in environmental flows assessments 101

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
River below Flaming Gorge Dam. In summary, the

studies discussed in the previous paragraphs provide

striking examples emphasizing that both flow and

thermal regimes must be managed simultaneously to

have the desired effects on riverine biota.

A key challenge in environmental flow assessments

is to determine the critical flow and thermal requirements of riverine ecosystems, while continuing to

provide the goods and services expected by human

society. Will there be conflicts between flow and

thermal targets in environmental flow prescriptions?

In the absence of a temperature control device, we

expect the answer to this question will be yes, quite

simply because stream temperature co-varies with

flow. For example, Krause et al. (2005) found that

proposed modifications to a regulated discharge

regime below Philpott Dam (Smith River, U.S.A.)

designed to improve physical habitat for brown trout

(Salmo trutta Linnaeus, 1758) would concurrently

reduce the frequency of optimal growth temperatures.

This study illustrated that the best management

option for improving flow regimes (e.g. for providing

critical fish habitat) was not necessarily the best for

improving thermal regimes (e.g. for providing optimal growth conditions). Consequently, different

aspects of the temperature regime for any stream

would require careful analysis before any environmental flow requirements could be safely prescribed.

Empirical data and predictive models are needed to

help identify and incorporate the potential interactive

effects of discharge and temperature recommendations in environmental flow assessments.

Synchronised management efforts are needed to

meet both flow and temperature requirements of

entire ecosystems by recoupling the natural hydrologic and thermal cycles. Given the complexities and

trade-offs regarding alternative dam strategies for

meeting both downstream flow and thermal standards, particularly in the absence of a temperature

control device, we believe the environmental flow

assessments would benefit from the use of formal

optimisation frameworks incorporated into adaptive

management strategies. Pareto efficiency (or Pareto

optimality), for example, is a central concept in

economics that is defined as the efficiency of a market

which is unable to produce more from the same level

of inputs without reducing the output of another

product. Recent efforts have developed Paretooptimal solutions for environmental flow schemes

that incorporate variability in flow regimes and

provide for human needs (e.g. Suen & Eheart, 2006;

Shiau & Wu, 2007), and we believe that the application of Pareto optimality holds promise for incorporating thermal criteria into environmental flow

assessments. For example, this multi-objective optimisation approach could be applied to identify a set of

flow schemes (i.e. those contained in the Pareto

frontier) that are considered efficient with respect to

meeting both the temperature and flow needs of a

riverine ecosystem while also ensuring the water

requirements of human users.

Pareto optimisation could account for the fact that

particular flow events that may be deemed critical for

riverine ecosystems (i.e. high discharge events for

mobilizing sediment and shaping channel forming

processes) may not be optimal for restoring critical

components of the thermal regime (i.e. high discharge

events can increase the magnitude and spatial extent of

coldwater pollution). To illustrate this, we examine the

Coosa River tailwater below Jordan Dam (U.S.A.),

where dam influences on tailwater temperatures

extend approximately 4 km downstream under low

flow regimes and nearly 15 km downstream under high

flow regimes (Jackson & Davies, 1988). In this instance,

environmental flow assessments that are considered

important for environmental flows are detrimental

with respect to thermal targets. By identifying a set of

efficient and acceptable environmental flows, an optimal solution could be selected to represent what would

be considered the optimal environmental flow prescription for a given river system. Relationships

between the conflicting flow and thermal objectives

would offer decision makers with the marginal tradeoffs useful for the selection of the preferred solutions. In

summary, there is continued need for the collection of

more empirical data and development of pragmatic

models that will provide the basis for adaptive management schemes involving both riverine flow and

thermal regimes (Rivers-Moore & Jewitt, 2007).

Challenge #5: Designing temperature-enlightened

environmental flow assessments in a changing climate

The prospect of dramatic climate change over the next

century underscores the need for adaptive management strategies for effectively designing and implementing environmental flows (Palmer et al., 2008).

Projected increases in air temperatures, combined

102 J. D. Olden and R. J. Naiman

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
with lower accumulation in winter snowpack, earlier

onset of spring peak flows, and lower summer

baseflows, will have direct implications for the thermal regimes of streams and rivers (Poff, Brinson &

Day, 2002). Equally relevant is the increasingly large

and thirsty human population which is rapidly

changing the geography of water demand and dramatically shifting current land use practices (Poff

et al., 2003; Fitzhugh & Richter, 2004), both of which

have direct and indirect implications for water temperatures. By failing to account for projected climate

change, ecologists run the risk of making flow

recommendations based on the characteristics of past

thermal regimes that are not favoured by present-day

or future environments.

Future climate change will significantly influence

how dams are operated to achieve environmental

flows that incorporate water temperatures. For example, Sinokrot et al. (1995) found that the predicted

impact of climate change on stream temperatures

below dams is more pronounced when water is

released from the reservoir surface compared to

deeper depths. Results from studies like this have

been used by some to advocate that management

agencies could use dams with deep, coldwater

releases to control thermal regimes in a manner that

offsets climate warming effects. We do not question

that innovative mitigation strategies are needed to

prepare for a rapidly changing climate; however,

we have a number of concerns with this proposed management approach. First, climate-induced

changes in stream discharge are expected to have

direct (i.e. thermal capacity) and indirect (i.e. changes

to channel geomorphology and riparian vegetation)

consequences for seasonal and annual patterns in

water temperature, which will be very difficult to

predict. Secondly, as previously discussed, hypolimnetic releases by dams from thermally stratified

reservoirs often result in abnormally warmer water

temperatures during the winter. This warming effect

is likely to be compounded in those regions predicted

to experience higher air temperatures during the

winter season; a trend already observed for some

rivers (Durance & Ormerod, 2009). Taken together, we

believe that our ability to mitigate the thermal

consequences of climate change through the management of dam operations is likely to be unsuccessful. In

summary, we urge ecologists to ground their knowledge in the past, but to look to the future in their

scientific endeavours. There is no doubt that environmental flow assessments will benefit greatly from

directly incorporating the predicted response of

freshwater ecosystems to projected changes in flow

and thermal regimes from climate change.

Prospectus

Recent decades have witnessed an increasing recognition of the importance of variability of ecosystem

drivers, such as river flow and water temperature in

maintaining river integrity. Even though there has been

tremendous progress in environmental flow research

and implementation, what remains unknown is formidable. Is more water alone sufficient without restoring

other essential water quality variables, such as recoupling the natural flow and thermal regimes? As we

continue to learn about these issues, clear and complete

answers remain elusive because of the complexity of

the scientific questions and management options.

Clearly, new strategies are needed that account for

the natural dynamics in temperature-related ecosystem

processes by using the natural thermal regime as one

template for environmental flow management.

From its name, the concept of ‘environmental flows’

would seem to offer the features of ecologically

sustainable water management, but in its current

form it has evolved to include only water quantity

and it is not comprehensive enough to solve the broad

water resource problems that will challenge us in the

future. We firmly believe that the benefit of using

environmental flows as a paradigm in water management will be its focus on the blending of both water

quantity and quality. For this reason, many additional

aspects of water quality must be recognised, including

pollutants, nutrients, organic matter and sediments

and dissolved oxygen (Nilsson & Reno ̈fa ̈lt, 2008),

which can interact with water temperature and

discharge in complex ways.

Our objective was to add clarity to the increasing

popular but still poorly understood concept of an

environmental flow. The issue is not that ecologists do

not recognise the importance of water temperature for

riverine ecosystems; rather to date we have not

formally integrated it into environmental flow assessments. This study began with an overview of the

natural thermal regime and its ecological importance, and ended with a discussion of some overarching research opportunities and challenges to

Incorporating thermal regimes in environmental flows assessments 103

2009 The Authors, Journal compilation 2009 Blackwell Publishing Ltd, Freshwater Biology, 55, 86–107
incorporating thermal criteria into the prescription of

environmental flows.

Based on our assessment, we suggest that scientific

research should focus on the ongoing collection of

long-term temperature records, 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 trade-offs and

provide a range of optimal environmental flows that

meet both ecosystem and human needs for fresh

water. Lastly, ecologists and water managers must

maintain a nimble capacity to incorporate results from

ongoing climate change research into adaptive management strategies involving environmental flows.

Acknowledgments

We thank participants of the 3rd International Symposium on Riverine Landscapes (South Stradbroke Island,

Queensland, Australia) for insightful conversations,

and Mark Kennard, Angela Arthington and Stuart

Bunn for their kind hospitality during our visit. We

gratefully acknowledge Steve Ormerod and Christer

Nilsson for comments on the manuscript, Bruce Webb

for discussions regarding the thermal impacts of dam

operation, David Archer, Mark Vinson and Daqing

Yang for providing temperature data, and Brad Sherman for generously providing the drawings in Fig. 5. J.

D. Olden thanks Land and Water Australia and the

Tropical Rivers and Coastal Knowledge (TRaCK)

research consortium for partial funding support, and

R. J. Naiman acknowledges the Freshwater Committee

of DIVERSITAS and the Global Water System Project. J.

D. Olden conceived and developed the idea for the

manuscript and conducted the data analysis, and J. D.

Olden and R. J. Naiman wrote the manuscript.

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