The effect of varied dam release mechanisms and storage volume on downstream river thermal regimes

Journal Pre-proof The effect of varied dam release mechanisms and storage volume on downstream river thermal regimes Laura E. Michie (Conceptualization) (Methodology) (Formal analysis) (Investigation) (Writing - original draft) (Writing - review and editing), James N. Hitchcock (Conceptualization) (Formal analysis) (Writing - review and editing), Jason D. Thiem (Writing review and editing), Craig A Boys (Writing - review and editing), Simon M. Mitrovic (Conceptualization) (Writing - review and editing) (Funding acquisition)

PII:

S0075-9511(19)30209-9

DOI:

https://doi.org/10.1016/j.limno.2020.125760

Reference:

LIMNO 125760

To appear in:

Limnologica

Received Date:

4 October 2019

Revised Date:

17 January 2020

Accepted Date:

5 February 2020

Please cite this article as: Michie LE, Hitchcock JN, Thiem JD, Boys CA, Mitrovic SM, The effect of varied dam release mechanisms and storage volume on downstream river thermal regimes, Limnologica (2020), doi: https://doi.org/10.1016/j.limno.2020.125760

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The effect of varied dam release mechanisms and storage volume on downstream river thermal regimes

Laura E. Michie 1 * 1

School of Life Sciences, University of Technology Sydney, PO

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Box 123, Broadway, New South Wales, 2007, Australia

* Corresponding author: [email protected] James N. Hitchcock 2

University of Sydney, Camperdown, New South Wales, 2006,

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Jason D. Thiem 3

Department of Primary Industries, Narrandera Fisheries

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Australia

Centre, PO Box 182, Narrandera, New South Wales, 2700,

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Australia Craig A Boys 4

NSW Department of Primary Industries, Port Stephens

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Fisheries Centre, Taylors Beach Road, 2316 Taylors Beach,

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NSW, Australia Simon M. Mitrovic 1 1

School of Life Sciences, University of Technology Sydney, PO

Box 123, Broadway, New South Wales, 2007, Australia

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Highlights 

At high storage volume, a novel thermal curtain could ameliorate thermal pollution by 8-10°C

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Thermal shock was caused by interchanging different dam release mechanisms

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High storage volume increases the magnitude of cold

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water pollution

Abstract

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Temperature plays an essential role in the ecology and biology of aquatic ecosystems. The use of dams to store and

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subsequently re-regulate river flows can have a negative impact

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on the natural thermal regime of rivers, causing thermal pollution of downstream river ecosystems. Autonomous thermal loggers were used to measure temperature changes

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downstream of a large dam on the Macquarie River, in

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Australia’s Murray-Darling Basin to quantify the effect of release mechanisms and dam storage volume on the

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downstream thermal regime. The degree of thermal pollution in the downstream river was affected by different release mechanisms, including bottom-level outlet releases, a thermal curtain (which draws water from above the hypolimnion), and spill-way release. Dam storage volume was linked to the magnitude of thermal pollution downstream; high storage 2

volumes were related to severe thermal suppressions, with an approximate 10°C difference occurring when water originated from high and low storage volumes. Downstream temperatures were 8 ̶ 10°C higher when surface releases were used via a thermal curtain and the spillway to mitigate cold water pollution that frequently occurs in the river. Demonstrating the effectiveness of engineering and operational strategies used to

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mitigate cold water pollution highlight their potential contribution to fish conservation, threatened species recovery

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and environmental remediation of aquatic ecosystems.

Key words: cold water pollution; environmental remediation;

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reservoir; temperature; thermal curtain; thermal pollution.

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1. Introduction

Temperature influences aquatic organisms in a range of key

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ways including metabolism (Gehrke & Fielder 1988), growth rates (Gehrke 1988; Koehn 2001; Ryan 2003), reproduction

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(Koehn & O'Connor 1990), spatial and temporal distribution (Peterson & Rabeni 1996) and community structure (Astles et al. 2003). Therefore, temperature plays an essential role in freshwater ecosystem health (Coutant 1999) and aquatic organisms have evolved to specific thermal regimes (Olden &

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Naiman 2010), which in rivers are driven by local atmospheric conditions, topography and stream discharge (Ward 1982).

Globally there are estimated to be >50,000 large dams (>15 m in height) which have impounded naturally flowing river systems (Lehner et al. 2011), with many more planned for construction in developing countries (Winemiller et al. 2016).

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Large dams have enhanced thermal energy storage and a higher thermal buffering capacity compared to rivers. The use of dams to store and subsequently re-regulate water for the purposes of

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hydropower, irrigation and drinking water can alter the natural thermal regime of rivers (Ward & Stanford 1983). In warm

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climatic regions, hypolimnetic water of thermally stratified

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dams can be significantly cooler than natural river temperatures (Weber et al. 2017). When this water is selected for release downstream it can cause thermal reductions to the downstream

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thermal regime; this effect is termed cold water pollution (Lugg & Copeland 2014). Cold water pollution has been documented

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downstream of large dams in many regions of the world

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including Australia (Lugg & Copeland 2014), China (He et al. 2018), America (Childs & Clarkson 1996; Clarkson & Childs 2000; Hart & Sherman 1996) and Europe (Slavik & Bartos 1997). Negative impacts on warm-water adapted fish from cold water pollution have been documented in a number of rivers (Astles et al. 2003; Clarkson & Childs 2000; Sherman et al.

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2007; Todd et al. 2005) and include reduced spawning success when required thermal cues are not met (Koehn & O'Connor 1990; Lake 1967), low post spawning survival (Todd et al. 2005), as well as inhibited growth rates (Astles et al. 2003) and swimming performance (Lyon, Ryan & Scroggie 2008).

The thermal impact of dams on downstream riverine habitats is

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variable, and is controlled by a number of interrelated factors including dam storage volume, the specific release mechanisms in place and the operation of these mechanisms. For example,

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many large dams release water from the lower water column

via bottom-level release outlets. As a result of stratification in

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the dam, downstream water temperatures can be suppressed by

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12–16 °C and the thermal impacts can extend over 100’s of kilometers (Burton 2000; Lugg & Copeland 2014; Vanicek 1970). Selective height withdrawal of water represents one

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potential management action to mitigate this issue, as the surface water of thermally stratified dams may closer resemble

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natural regimes (Sherman 2000). Multi-level outlets have been

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successful in mitigating thermal pollution (Du Xiaohu 2009; Wu et al. 2011), but are an expensive retrofit to older dams (Sherman 2000). Cheaper infrastructure options such as trunnions and thermal curtains have been trialed, and these aim to reduce cold water pollution by diverting surface waters to the bottom-level outlet (Gray et al. 2019), although their utility in

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the restoration of natural thermal regimes in rivers requires more thorough investigation. The volume of water also influences the mechanism selected for water release; above full capacity operators may require the use of the spillway, thus releasing surface water downstream that more closely resembles natural thermal regimes than water released from a bottom-level outlet.

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We investigated the interrelated effect of dam storage volume, specific release mechanisms and the subsequent operation of these mechanisms on the degree and extent of cold water

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pollution on the Macquarie River downstream from

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Burrendong Dam, Australia. This dam represents one of 10 large dams in the Murray-Darling Basin considered to have a

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substantial negative effect on downstream water temperatures and associated ecosystem processes, and has been prioritized

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for restoration (Lugg & Copeland 2014). Burrendong Dam was originally constructed with a bottom-level release outlet and a

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spillway. A thermal curtain was retrospectively fitted in 2014 to enable diversion of water from the surface layers of the dam to

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the bottom outlet to be released downstream (Gray et al. 2019). Over a five year period (2013-2018) dam storage volume varied substantially (10–140%), and as a result we sought to quantify 1) how different mechanisms of water release from dams affect downstream temperature, and 2) how the storage volume of dams affects downstream temperature. Given the 6

number of large dams in operation globally causing potential cold water pollution, and the negative effects they have upon important ecosystem processes downstream, we demonstrate that management solutions that include engineering and operational strategies can result in substantial environmental

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remediation of aquatic ecosystems globally.

2. Methods

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2.1 Study area

Burrendong Dam (-32.668, 149.109) is a large dam on the

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Macquarie River in central-western New South Wales (NSW),

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Australia. It receives inflows from the Macquarie and Cudgegong Rivers from a combined catchment area of 13 900 km2; outflows join the Macquarie River, which eventually

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forms a confluence with the Barwon River and then the Darling River. Burrendong Dam is the 7th largest dam in NSW,

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Australia. At full supply level it sits at 344 m above mean sea

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level and holds 1188 GL, with an additional 489 GL of storage reserved for flood mitigation. The maximum depth of the storage is 57 m, covering an area of 7200 hectares. Volumes of releases are usually highest in spring and summer to meet demands for water supply for domestic, irrigative and stock requirements and environmental water for the Macquarie

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Marshes (Green et al. 2011). Releases from the storage may occur via a bottom-release outlet, a spillway and a thermal curtain. At full supply level, the bottom-release outlet sits 31.4 m below the surface of the water and the spillway releases surface water. The depth of the thermal curtain within the water column can be manipulated and is usually maintained

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between 7–8 m below the surface.

Figure 1: Map of thermal monitoring locations on the Macquarie River, showing the location of Burrendong Dam (-32.668, 149.109). Three downstream monitoring sites are indicated (black dots) at 7.5 km, 33 km and 55 km downstream of Burrendong Dam (labeled). The dark shaded area of the insert indicates the spread of the Macquarie River catchment.

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2.2 Thermal curtain at Burrendong Dam A novel thermal curtain was installed at Burrendong Dam in 2014 for the purposes of mitigating cold water pollution in the downstream Macquarie River (Gray et al. 2019). The thermal curtain is a flexible, reinforced polypropylene fabric which is suspended from the original outlet tower. Cylindrical in shape,

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it encircles the outlet tower to funnel water from the top opening of the curtain to the existing bottom-level outlet for release downstream (Figure 2). The curtain is secured to the lake bed to prevent hypolimnetic water entering the curtain

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structure from the bottom of the water column. The height of

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the top opening can be manipulated via the cables that suspend the curtain from the outlet tower. This functionality enables

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dam managers to adjust the depth of the curtain with variation in the depth of the reservoir and enables active selection of

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water depth for release downstream. Typically the thermal curtain is maintained at 7 m below the surface of the dam when

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it is used for cold water pollution mitigation, withdrawing water from the epilimnion. This can be deeper if an algal bloom

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accrues in the surface of the dam to increase hypolimnial contribution and reduce downstream algal concentrations.

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Figure 2: A simplified schematic of the operation and structure of the thermal curtain in place at Burrendong Dam.

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2.3 Operational scenarios evaluated

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Storage volume, release volume, mode of releases and depth of releases were assessed at Burrendong Dam over a five-year

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period (2013-2018). Storage volume, water level and discharge of Burrendong Dam were obtained from gauging stations

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operated by WaterNSW (http://realtimedata.waternsw.com.au). The depth of releases was calculated from reservoir water level.

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These parameters were calculated as daily averages from measurements taken at 15-minute increments. WaterNSW staff

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at Burrendong Dam provided information on the operation of release methods used.

2.4 Temperature data Temperature data was obtained from a gauging station 7.5km downstream and from temperature loggers (HOBO Pendant 10

TM) installed 33 km and 55 km downstream of Burrendong Dam (Figure 1). At gauging stations temperature was measured at 15-minute increments and made available through WaterNSW (http://realtimedata.waternsw.com.au). Temperature loggers recorded temperature at 30-minute intervals; data was downloaded from the field sites approximately every 6 months. Daily average temperatures

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were calculated from this data. Warm peaks were measured when water temperatures were

measured above 20 °C, this temperature was used as a thermal

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threshold as this temperature is optimal for the spawning of

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Murray Cod (Lake 1967; Rowland 2004), with slightly warmer temperatures necessary for Golden Perch and Silver Perch

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(Lake 1967). These species of fish are native to the region, with populations being affected by cold water pollution (Lugg &

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Copeland 2014).

The effect of storage volume upon water temperature released

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downstream via the bottom-release outlet was estimated for two time periods. Two years of variation in storage volume were

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chosen for a whole summer analysis, monthly average temperatures were calculated from 15-minute measurement intervals from December to April in both 2013-2014 and 20162017. February was used in a 20-year analysis of storage volume and downstream temperature. Linear regression analysis was performed to determine if the monthly average 11

storage volume (%) was a significant predictor of the monthly average temperature (⁰ C) at P = 0.05. Two years (2015, 2016) were excluded from this data set as the thermal curtain was in use. The change in use of release mechanisms from the spillway to the bottom-release outlet through time was plotted against temperatures measured at three distances downstream. The

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temperatures plotted were calculated as daily averages from 15minute measurement intervals.

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3. Results

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3.1 Effect of release mechanism on downstream

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temperature

Outflows from Burrendong Dam between 2013 and 2018

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occurred via a variety of release mechanisms in place at the dam (Figure 3a). The bottom-level outlet was used throughout

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2013 and early 2014. On the 7th of May 2014 a thermal curtain was installed withdrawing water from approximately the top 7

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m of the water column. The curtain was the sole method of release until the 13th of September 2016, when releases commenced via both the curtain and the spillway as the dam reached 136% storage volume. After the 4th of November releases occurred only via the spillway, and this was maintained until the 1st of December, followed by a four-day 12

period where releases were transitioned from the spillway to the bottom-level outlet, which was then used for the remainder of the study period. The varied modes of releases from Burrendong Dam occurred from a range of depths within the water column (Figure 3b). Outlet depth varied as storage volume changed within Burrendong Dam, in 2013, the outlet released water from a

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depth of approximately 20 m. As storage volume decreased

over the 2013/2014 summer period (Figure 3a), the depth of the outlet was approximately 10 m below the surface of

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Burrendong Dam. The thermal curtain was implemented in

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May 2014 and water was withdrawn for release from

approximately the top 7 m of the water column, whilst the

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storage volume of the dam was still low. These releases were maintained till late 2016. Water was released via the spillway

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(top 1 m) for a brief period in late 2016 before releases were transitioned to the bottom-level outlet. Storage volume within

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Burrendong Dam at this time was approximately 120%, with the outlet being 32 m below the surface of the dam. As storage

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volume decreased in Burrendong Dam throughout 2017, outlet depth was gradually reduced to 25 m. Temperature of outflows varied between years, as different mechanisms of releasing water from Burrendong Dam were used and water was withdrawn from different depths (Figure 3c). Outlet releases from January 2013 to April 2014 resulted in 13

two summer temperature peaks of approximately 24 °C in late February of each year. In the 2012-2013 summer period, seasonal warm peaks (average temperature above 20 °C) lasted for 42 days, first attained on the 24th of February 2013. In the 2013-2014 summer this was 101 days commencing on the 7th of January 2014. Summer thermal peaks during curtain releases (2014-2015 and 2015-2016) were slightly warmer at

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approximately 25 °C also occurring in late February. Warm peaks were longer, lasting 142 days (2014-2015) and 171 days (2015-2016), with temperatures above 20 °C first attained in

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early November.

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In late 2016, epilimnial waters were released from the dam between August and December, firstly through the thermal

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curtain and then via the spillway. Water temperature above 20 °C was first attained downstream on the 11th of November, and

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peaked at 22 °C in early December whilst releases occurred via the spillway (Figure 2c). Water temperatures downstream of

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Burrendong Dam drop by approximately 10 °C as releases were transitioned from the spillway to the bottom outlet in early

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December and were between 13 °C and 14 °C for the remainder of summer, being considerably cooler than bottom outlet releases that occurred during 2013 and 2014. In early March 2017, extremely low volumes of releases from Burrendong Dam (Figure 3a) coincide with an increased spike in temperature downstream of the dam. 14

ro of -p re lP na ur Jo Figure 3: a) Plot of storage volume, storage release volumes and mode of releases at Burrendong Dam over a 5 year period (2013-2018). b) Depth of varied release mechanisms in place at Burrendong Dam. c)

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Daily average water temperature downstream of Burrendong Dam (7.5 km) over a 5-year period (2013-2018). Background color above the Xaxis on all figures shows release mechanism used; white background is bottom outlet releases, light grey is thermal curtain releases and dark grey is spillway releases.

Temperature was assessed at various sites downstream of Burrendong Dam from October 2016 to April 2017 (Figure 4)

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when releases were transitioned from the spillway to the bottom-outlet (Figure 3a). At the site immediately downstream (7.5km) of Burrendong Dam, temperature gradually increased

through late 2016 from October, reaching approximately 22 °C

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in late November. In early December water temperature was

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reduced by almost 10 °C, corresponding with the transition from spillway to bottom-outlet releases (Figure 4a). Water

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temperature downstream of Burrendong Dam was maintained at approximately 12 – 14 °C until mid-March.

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Further downstream (33 km & 55 km) of Burrendong Dam water temperature also gradually increased from October to

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December 2016, reaching approximately 24 °C in late

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November. Temperature reductions were observed at both sites in early December, correlating temporally with the change in release modes that occurred at this time from Burrendong Dam. A 9 °C reduction in temperature was recorded at the site 33 km downstream, whilst a 7 C reduction occurred 55 km downstream of the dam.

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Figure 4: Daily average water temperature at three sites (a) 7.5 km, b) 33 km & c) 55 km) downstream of Burrendong Dam from October 2017 to April 2017. Background color indicates release mechanism used with white indicating bottom-level outlet and dark grey indicating the mixed use of the spillway and thermal curtain.

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3.2 Effect of storage volume on downstream temperature The storage volume varied in Burrendong Dam considerably, ranging from a minimum of 10% to a maximum of 136% (Figure 3a). Throughout 2013, a decline from 60% to 20% total storage volume occurred, following which the dam was

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maintained at approximately 10-20% from 2014 to June 2016. Extended periods of heavy rainfall throughout June to

September 2016 in the Macquarie catchment saw Burrendong

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Dam increase rapidly in storage volume from 11% on the 1st June 2016 to 137% on the 23rd of September (114 days).

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Volumes of releases were generally higher during spring and summer but were highest when releasing via the spillway while

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the dam was over 100% storage volume.

Storage volume varied considerably in Burrendong Dam

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between 2013 and 2018, with a minimum volume of about 10% and a maximum of 135% (Figure 3a). Whilst releases occurred

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via the bottom-level outlet, temperature was assessed

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downstream of Burrendong Dam for two summers with different storage volumes (Figure 5). In the summer of 2013/14 Burrendong Dam had low-moderate storage volume, ranging between 20 and 30% whereas the summer of 2016/17 storage volume was high and ranged between 100 and 120%. Water temperatures were much warmer in 2013/14 when the dam was

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at low storage volume, temperature peaked at approximately 23 °C through February and March. High storage volume releases in 2016/17 were much cooler with temperatures being approximately 14 °C through December, January and February. The greatest difference in temperatures across these two 12 month periods occurred in February with an 8.75 °C variance between bottom-outlet releases at high and low storage volume.

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A significant linear relationship (r2=0.922 p<0.001) between storage volume of Burrendong Dam and temperatures

downstream illustrates that increased storage volume increases

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the severity of thermal pollution downstream with the bottomoutlet in use (Figure 6). Temperatures downstream under low

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storage volumes were between 22 and 26 °C whereas under

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high storage volume conditions downstream temperatures were

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considerably lower between 12 and 16 °C.

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Figure 5: Average monthly temperature downstream of Burrendong Dam

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(7.5km) from December to April. High (2016/17) and low (2013/14) storage

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volumes are compared. Error bars are standard error of the mean.

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Figure 6: Storage volume and temperature downstream of Burrendong

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Dam (7.5 km) Measurements are averages for the month of February in

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years that bottom-outlet releases occurred between 1998 and 2018.

4. Discussion

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By monitoring the interrelationship between dam release mechanisms, storage volume and operation of a large dam in

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Australia’s Murray-Darling Basin we demonstrated the benefit

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of engineering solutions and adaptive operation on the downstream thermal regime of a large river. Due to a full supply level and the use of a bottom-offtake, the magnitude of cold water pollution in the Macquarie River was high in summer. Implementation of warm surface releases via a thermal curtain and the spillway mitigated this impact, but

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when releases transitioned between surface and offtake a rapid reduction in temperature (cold shock) occurred downstream.

There are >50,000 large dams (taller than 15m) in the world. Whilst studies identifying thermal pollution downstream of large dams are rare on a global scale (for example (Lavis &

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Smith 1972; Lehmkuhl 1972; Zhong & Power 1996)) It is highly likely that thermal pollution will be a problem

downstream of many more dams than those that have been

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assessed (Lehner et al. 2011). While thermal pollution of

freshwater rivers is acknowledged as a global problem that

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negatively affects the survival and physiology of aquatic biota (Donaldson et al. 2008; Sylvester 1972), there remain few

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effective examples that document solutions to address this problem (Sherman 2000). In Australia’s Murray-Darling Basin,

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10 large dams are the cause of approximately 2000 km of collective thermal pollution of rivers (NSW-CWPIG 2012).

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With a temperate to semi-arid climate, this region experiences

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thermal pollution when bottom-offtake releases of cold hypolimnetic water occur from thermally stratified dams, with effects extending to downstream riverine habitat for as much as 300 km in a single river (Burton 2000; Lugg & Copeland 2014). In the Glen Canyon Dam on the Colorado River, USA, it is estimated bottom release lead to thermal pollution up 930 km 22

downstream (Stevens, Shannon & Blinn 1997). There is no current estimate of the length of rivers globally affected by thermal pollution. This highlights the need to not only fully describe the problem of thermal pollution but to rapidly validate and put into practice management solutions to mitigate negative impacts. While the implementation of a thermal curtain could mitigate

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cold water pollution, adaptive dam operation is required to

realize the full benefit. At low storage volume, during winter and above full storage volume while spillway releases occur

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there is limited benefit of the thermal curtain. At low storage

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volume the thermal curtain was estimated to increase

downstream river temperatures by approximately 3 °C (Gray et

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al. 2019), although we propose that at high storage volume river temperatures could be increased by as much as 8–10 °C.

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Further impacts of bottom-release dams upon river thermal regimes include delayed seasonal peaks, reduced diel variation

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and overall reduction in annual amplitude (Lugg & Copeland 2014; Sherman 2000; Todd et al. 2005). Seasonal thermal

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peaks above 20 °C persisted for longer with the thermal curtain, creating favorable conditions for native fish breeding in the region as a number of native fish species depend on thermal thresholds of approximately 20 °C for successful spawning (Lake 1967; Rowland 2004). In the year post implementation of the thermal curtain at Burrendong Dam, water temperatures 23

downstream of the dam met thermal cues required for Murray cod spawning (approx. 20 ⁰ C) for 54% of their expected spawning season; this was 0% in the year prior to its implementation (Gray et al. 2019). In addition, annual amplitude was higher with warmer temperatures observed in summer. Warmer temperatures attained through the use of engineering solutions such as thermal curtains could eventuate

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in positive outcomes for native fish populations in downstream ecosystems (Clarkson & Childs 2000; Ryan 2003; Todd et al. 2005).

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Alternative engineering solutions have been implemented at

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other dams to mitigate the effects of dams on downstream thermal regimes. Successful mitigation of cold water pollution

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has occurred at Tallowa Dam, Australia where destratification of the water column was achieved through installation of an

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aeration system. Temperature increases resulted in increased diversity and abundance of fish (Miles & West 2011). Multi-

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level outlets are most commonly used to mitigate thermal pollution, and have been used successfully in China (Gao et al.

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2010; Wu et al. 2011) and America (Du Xiaohu 2009; Olden & Naiman 2010). They have been proposed as possible mitigation options after being modelled to be successful in Germany (Weber et al. 2017), China (He et al. 2017), and Australia (Sherman et al. 2007). Multi-level outlets are expensive to retrofit to dams after construction, and the environmental 24

benefits attained through their use are often not valued against the upfront costs (Sherman 2000). When considering Burrendong Dam for mitigation of cold water pollution, a multi-level outlet was estimated to cost $25 million (Sherman 2000), with the thermal curtain only costing $4 million it provided a much more viable option if successful. Spillways release surface waters of dams, and are implemented

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during floods when the capacity of other release mechanisms in place at the dam are exceeded (Acreman 2000). Spillway

releases occurred at Burrendong Dam whilst storage volume

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was above 100%; releasing surface water that was close to

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natural expected river temperatures. Thermal conditions of surface waters of large dams are more representative of natural

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river temperatures (Weber et al. 2017), but problems arise as toxic algal blooms often accrue in the surface waters of dams

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and may be spread downstream if this water is selected for release (Asaeda et al. 2001; Asaeda et al. 1996; Paerl &

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Huisman 2008; Priyantha et al. 1997). Operational changes between the varied release mechanisms at

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Burrendong Dam affected downstream water temperatures. A transition from spillway to outlet releases whilst the dam was thermally stratified resulted in a 10 °C decrease in temperature occurring 7.5 km downstream. Temperature reductions also occurred further downstream. Thermally stratified dams can have discernable temperature differences between surface 25

waters and bottom waters; temperature differences from 8 to 16 °C have been reported both in Australia (Baldwin et al. 2008; Lugg & Copeland 2014; Preece & Jones 2002; Sherman 2005) and internationally (Bonnet, Poulin & Devaux 2000; Hart & Sherman 1996). A model developed to predict temperatures of water released from varied outlet locations of the Grangent reservoir, France, predicted that outflow temperature would

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drop by 14 °C if releases were transitioned from the surface to the bottom of the water column whilst the reservoir is in a

stratified state (Gaillard 1984). These fast reductions in water

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temperature are often referred to as cold shock, and are often overlooked in water management plans that address thermal

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pollution (Donaldson et al. 2008; Ryan & Preece 2003).

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Inter-annual variation in dam condition, hydro-meteorological conditions and operation schemes require long-term data sets to

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infer an accurate impact of dams (Webb & Walling 1993, 1996). Analysis of 20 years of historical data demonstrated that

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temperature downstream of the dam was affected by storage volume, with outlet releases at higher storage volumes resulting

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in substantially cooler temperatures downstream. The impact of storage volume on thermal pollution in Australia has not been greatly addressed, despite the wide and often rapid changes in volume that can occur due to a climate defined by highly variable rainfall. Internationally, storage volume has been linked to the magnitude of downstream thermal pollution (He et 26

al. 2018; Yates et al. 2008). In order to develop a model to determine water temperature of water discharged from large dams in China, He et al. (2018) found water temperature to be higher at lower dam storage volumes. As storage volume is reduced, the thermocline shifts closer to the intake and warmer water in the upper layer of the water column becomes incorporated into the withdrawal zone. Storage volume was

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also deemed a defining factor for the temperature of water being released from the Shasta Reservoir, Sacramento River, California, U.S.A (Yates et al. 2008). Increased storage

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volumes in the Shasta Reservoir equated to cooler water being released; at storage volumes below 1480 GL release

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temperature was prescribed at 15 °C, whereas above 2700 GL

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this was reduced to 11 °C (Yates et al. 2008). Larger volumes of water have an enhanced facility for energy storage and have a higher thermal buffering capacity, thus having more severe

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thermal effects upon downstream thermal regimes. We demonstrate that mitigation of thermal pollution is more

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imperative downstream of large dams that frequently have high

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storage volumes or may have large and rapid fluctuations in storage volume.

5. Conclusions

Little systematic data exists to detail the global extent and occurrence of thermal pollution downstream of more than

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50,000 large dams that currently exist. Existing data does however point towards serious problems for the health of aquatic ecosystems and fish populations (Lugg & Copeland 2014; McCartney 2009). Mitigation structures that withdraw surface waters for release may reduce cold water pollution impacts downstream, although adaptive dam operation is required to realize full benefits and avoid causing the spread of

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algal blooms and cold shock downstream. These results provide important knowledge for the potential efficacy of different management interventions. We note that because of the

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complex set of interrelating factors including climate, dam size

and infrastructures, and water use, the potential approaches will

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need to be unique for each dam. Given the extensive nature of

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river regulation and thermal pollution worldwide, effective and economic mitigation strategies are essential in the remediation of aquatic ecosystems globally. Perhaps importantly, these

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results should serve as a reminder for the regions across the globe where large dams are still being built of the difficulty in

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mitigating thermal pollution retrospectively rather planning

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during dam construction.

Author statement

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Laura E Michie: Conceptualization, Methodology, Formal analysis, Investigation, Writing – Original Draft, Writing – Review & Editing. James N Hitchcock: Conceptualization, Formal analysis, Writing – Review & Editing. Jason D Thiem: Writing – Review & Editing. Craig A Boys: Writing – Review & Editing. Simon M Mitrovic: Conceptualization, Writing – Review & Editing, Funding acquisition

Conflict of interest

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Acknowledgments

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To the best of the knowledge of the authors, no conflict of interest, financial or other exists. We have included the relevant acknowledgements and funding sources.

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This research was supported by DPI Water and DPI Fisheries. The authors would like to thank Matthew Balzer for field

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assistance, and WaterNSW staff at Burrendong Dam for providing necessary data on the operation of Burrendong Dam

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and assisting in field visits.

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