Earth-Science Reviews 146 (2015) 77–91

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Earth-Science Reviews

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Environmental consequences of damming the mainstream Lancang- River: A review

Hui Fan a,b,⁎,DamingHea,b,HailongWangc a Asian International Rivers Center of University, 650091, b Yunnan Key Laboratory of International Rivers and Transboundary Eco-Security, Kunming 650091, China c Huaneng Lancang River Hydropower Company Ltd., Kunming 650214, China article info abstract

Article history: Damming rivers to generate hydropower can help mitigate the world's energy crisis and reduce the risk of global Received 12 July 2014 climate change; however, damming can also produce enormous negative effects on the environment and ecosys- Accepted 27 March 2015 tems. The mainstream Lancang-Mekong River within China has been planned as one of the thirteen state hydro- Available online 3 April 2015 power bases. To date, there have been six operational along the mainstream Lancang River, and the 15 remaining dams of the proposed Lancang cascade will be completed in the next decades. In this paper, we exam- Keywords: ined several crucial environmental changes and ecological responses that have resulted from the construction construction Environmental impacts and operation of the existing dams of the Lancang cascade. The current literature and observational data suggest Ecosystem responses that the commissioned dams have led to a decline in the flood season water discharge and annual sediment flux Lancang cascade dams within China's borders, reservoir aggradations, and water quality degradation in the reservoirs, which has nega- Mekong River tively affected riverine aquatic biological communities and fish assemblages. In contrast, the dams have only had Transboundary waterway small unfavorable effects on downstream environments and ecosystems outside of China. Because of the poten- tial environmental and geopolitical risks of the Lancang cascade dams, a long-term basin-wide terrestrial and aquatic monitoring program is urgently required to ensure that regional sustainable development occurs in the Lancang-Mekong River Basin. © 2015 Elsevier B.V. All rights reserved.

Contents

1. Introduction...... 77 2. ProfileoftheLancangcascadedams...... 78 3. Post-damenvironmentalconsequences...... 80 3.1. Hydrologicaleffects...... 80 3.2. Upstreamreservoirsiltationanddownstreamsedimentdischargedecline...... 80 3.3. Changesinwaterquality...... 83 4. Ecologicalconsequencesofdam-triggeredenvironmentalchanges...... 84 4.1. Responsesofphytoplanktonandzooplankton...... 84 4.2. Responsesofthezoobenthos...... 86 4.3. Responses of fishresources...... 88 5. Concludingremarks...... 89 Acknowledgments...... 89 References...... 89

1. Introduction

Climate change is one of the greatest challenges of the 21st century (IPCC, 2011), and mitigating rising global surface temperatures requires ⁎ Corresponding author at: Asian International Rivers Center of Yunnan University, No.2 North Cuihu Road, Kunming, Yunnan 650091, China. Tel./fax: +86 871 65034577. additional renewable energy sources to satisfy the increasingly high pri- E-mail address: [email protected] (H. Fan). mary energy demand, which is driven by the global population and

http://dx.doi.org/10.1016/j.earscirev.2015.03.007 0012-8252/© 2015 Elsevier B.V. All rights reserved. 78 H. Fan et al. / Earth-Science Reviews 146 (2015) 77–91 economic growth. As the second largest renewable energy vector and respectively, is underway (Yuan, 2010). Of the planned cascade dams largest source of renewable energy in the electricity sector, hydropower along the mainstream Lancang River, there are currently six large has a significant potential to reduce anthropogenic greenhouse gas commissioned dams: the Manwan, Dachaoshan, Jinhong, Xiaowan, emissions. In 2012, hydropower was estimated to account for approxi- Gongguoqiao, and Nuozhadu dams, with the Nuozhadu completed in mately 3.8% of the world's primary energy use and contribute approxi- 2013. According to the hydropower development plan proposed by mately 16.5% to the global electricity supply (REN21, 2014; Spänhoff, the Huaneng Lancang River Hydropower Company (also known as 2014). By the end of 2013, the total global installed hydropower capac- Hydrolancang), the remaining 8 dams of the planned cascade in the ity increased to 1000 GW, producing an annual power generation of Yunnan section of the Lancang River will be nearly operational by 3750 TWh (REN21, 2014). However, the total worldwide technical hy- 2016, and the hydropower installed capacity in the Lancang mainstream dropower potential and annual average energy generation are estimat- will total 30.0 GW by 2020 (Yuan, 2010). Several problems, such as land ed at 3721 GW and 14 576 TWh, respectively, which are roughly four inundation (He et al., 2004), hydrologic regime alteration (He et al., times the current hydropower capacity and generation (IPCC, 2011; 2006; Lu and Siew, 2006), sediment trapping (Fu and He, 2007; REN21, 2014). Of the total technical potential for hydropower, the unex- Kummu and Varis, 2007; Fu et al., 2008a,b; Kummu et al., 2010), ploited capacity varies from approximately 47% in Europe and North reservoir-triggered seismicity (Li et al., 2004, 2012), geological instabil- America to 92% in Africa (IPCC, 2011; REN21, 2014). This indicates ity (He et al., 2004), habitat fragmentation (Yi et al., 2014), and resettle- that there are large-scale opportunities for continued hydropower de- ment (Wang et al., 2013), have emerged with the unprecedented velopment worldwide, with the largest development potentials in advancement of extensive hydropower development. In particular, the Africa, Asia and Latin America. Lancang-Mekong River is one of the largest Asian international rivers China is a rapidly developing country with heavy energy consump- and a hot spot for biodiversity, and it links China with five downstream tion. Although China is ranked first in the world in terms of both total Southeast Asian countries: Myanmar, Laos, , Thailand and installed hydropower capacity and generation, the share of its hydro- Vietnam; thus, the river has extremely high ecological, economic power towards the total electricity production and the development and sociological importance. Although only a small fraction of the level of its hydropower are relatively low (Huang and Yan, 2009). By Lancang-Mekong's discharge is input to the South China Sea from the the end of 2013, China's installed hydropower capacity and energy gen- upstream Chinese catchment, the downstream transboundary potential eration totaled 280 GW and 896.3 TWh, respectively, accounting for ap- impacts of hydropower development have attracted much attention proximately 22.4% and 16.8%, respectively, of China's total (Hu, 2014). since China planned the Lancang dam cascade along the middle and The share of hydropower in the total electricity production is only lower sections of the Lancang River. Hydrological effects with respect 17.2%, which is much less than the shares of 96.6%, 83.7% and 57.9% in to reservoir filling and water release have been maliciously exaggerated Norway, Brazil and Canada, respectively (Huang and Yan, 2009). To because of the scarcity of reliable data and knowledge (Lu and Siew, meet its goal of cutting carbon dioxide emissions per unit of gross do- 2006; Campbell, 2007; Adamson et al., 2009; Lu et al., 2014). This mestic product (GDP) by 40–45% before 2020, China will have to in- poses the most important threat to strengthening the geopolitical and crease its efforts to develop hydropower. Therefore, China announced economic cooperation in the Greater Mekong Subregion (GMS). To to mainly construct eight of the 13 planned hydropower bases eliminate much of the apprehension and discontent over the potentially (i.e., , Lancang River, , Upper Huanghe River, Ya- adverse impacts of extensive hydropower development on the local and , Nu River, mainstream Changjiang River, and Middle Yarlung downstream environment, the environmental consequences of the Zangbo River) during the Twelfth Five-Year period (2011–2015) and commissioned dams along the Lancang mainstream must be examined further expand its installed conventional hydropower capacity to in detail. Therefore, the present study aims to provide a comprehensive 420 GW by 2020, which includes the current conventional hydropower overview of the environmental effects of damming in the mid-lower of 350 GW (CNEA and CNREC, 2012). This plan explicitly listed the mainstream of the Lancang River, and the planning and construction 60 key hydropower projects during the Twelfth Five-Year period, 13 of dams along the mainstream Lancang River are introduced first. We of which will be built along the mainstream Lancang River (CNEA and then analyze the environmental effects of the Lancang cascade dams CNREC, 2012). on the hydrologic regime, water quality, sediment trapping, reservoir The Lancang River is the upper section of the 4880-kilometer-long siltation, and aquatic fauna (phytoplankton, zooplankton, zoobenthos Lancang-Mekong River, which is the largest river in Southeast Asia and fish resources). Finally, we summarize several aspects of the chal- and has a drainage basin of approximately 795 000 km2 (Campbell, lenges and problems that must be solved and offer suggestions for fu- 2009). It rises in the Guozongmucha Mountain in Zaduo County, ture environmental monitoring and research of the Lancang cascade Province, where the water source has an altitude of 5244 m reservoirs. and is located at 94°41′44″E and 33°42′31″N(Zhou and Guan, 2001). The drainage area is 167 487 km2 and 2161 km long; the mean annual 2. Profile of the Lancang cascade dams discharge is 2180 m3/s (Zhou and Guan, 2001). From the source to the outlet of the river on the China–Myanmar border, the river plunges The mainstream Lancang River located downstream of Changdu in 4700 m through the high gorges of the Xizang Autonomous Region the Xizang Autonomous Region can be roughly divided into two parts: and Yunnan Province, which is more than 90% of its entire drop in ele- the Xizang section and Yunnan section (Fig. 1). Along the Xizang sec- vation. Accordingly, the Lancang River is substantially abundant in hy- tion, a six-dam cascade, which includes the Cege, Yuelong, Kagong, dropower resources. The 4th national survey of hydropower resources Banda, Rumei and Guxue dams, with a total installed capacity of that ended in November 2005 indicates that the total theoretical hydro- 5.9 GW and annual average energy generation of 28.8 TWh is projected power potential and annual average energy generation in the Lancang (Fig. 1)(Yuan, 2010). The construction of these dams will begin after River Basin are estimated at 35.9 GW and 314.4 TWh, respectively (Li 2015 and will be completed by 2030. The Yunnan section of the main- and Shi, 2006; Yuan, 2010). The technically exploitable installed capac- stream Lancang River is approximately 1240 km long and has an eleva- ity and annual average energy generation have been estimated at tion drop of 1780 m (Zhou and Qian, 2011). Along the upper channel of 34.8 GW and 169.0 TWh, respectively (Li and Shi, 2006; Yuan, 2010). the river section, a cascade of seven dams, which includes the Gushui, To fully exploit the river's resources, several mega-schemes to dam Wunonglong, Lidi, Tuoba, Huangdeng, Dahuaqiao and Miaowei dams, the Lancang River's mainstream for generating electricity have been with a total installed capacity of 8.2 GW and annual average energy gen- proposed since the 1950s (Zhao, 2000). At present, a cascade hydro- eration of 40.0 TWh is planned (Fig. 1)(Ma, 2004; Zhou and Qian, development scheme of 21 dams with a total installed capacity and 2011). Of these dams, most are presently underway and will be com- annual average energy generation of 32.4 GW and 145.0 TWh, pleted by 2016 (Yuan, 2010). In the middle and lower portions of the H. Fan et al. / Earth-Science Reviews 146 (2015) 77–91 79

a) b)

c)c)c)

Fig. 1. The Lancang-Mekong River Basin and Lancang cascade dams. a) Location of the Lancang-Mekong River Basin; b) locations of the Lancang cascade dams and major gauging stations; and c) longitudinal profile of eight cascading large dams along the middle and lower reaches of the Lancang River. Source: HydroChina, 2010; Zhao, 2000.

Yunnan section, an eight-dam cascade is proposed to exploit an Although the primary scheme of hydropower development on the 828 meter drop in elevation along the 772 kilometer river channel (Li, Lancang River was proposed in the 1950s, the first dam of the Lancang 1988). The total installed capacity and annual average energy genera- cascade, the , was not constructed until 1986. Over the tion of these dams, such as the Gongguoqiao, Xiaowan, Manwan, last three decades, dam construction along the Lancang River advanced Dachaoshan, Nuozhadu, Jinghong, Ganlanba and Mengsong (Fig. 1), rapidly; by 2012, six large mainstream dams were operational (Table 1). reached 16.2 GW and 73.0 TWh, respectively (Li, 1988). Upon the clo- The Manwan Dam began operating in 1993 and became fully operation- sure of 15 dams in the Yunnan section, the total and active reservoir al in 1996, and another five dams began successive operations: storages of these cascade reservoirs amounted to 532 × 108 m3 and in 2001, in 2008, in 291 × 108 m3, respectively (Ma, 2004). 2009, Gongguoqiao Dam in 2011, and in 2012. Together, 80 H. Fan et al. / Earth-Science Reviews 146 (2015) 77–91

Table 1 Key indicators of the six commissioned dams of the Lancang dam cascade. Source: Zhao, 2000.

Dam Catchment Annual Normal storage Dam Total Active Installed Guaranteed Annual Reservoir area inflow water level height storage storage capacity capability generation filling

(km2)(m3/s) (m) (m) (108 m3) (108 m3) (MWh) (MW) (104 MWh)

Gongguoqiao 97 300 985 1319 130 5.10 1.20 750 390 406 Sep. 2011 Xiaowan 113 300 1220 1240 292 151.32 98.95 4200 1854 1889 Dec. 2008 Manwan 114 500 1230 994 132 10.60 2.57 1500 807 781 Mar. 1993 Daochaoshan 121 000 1340 899 120.5 8.84 3.67 1350 712 670 Nov. 2001 Nuozhadu 144 700 1750 812 260 223.68 121.95 5500 2403 2378 Nov. 2011 Jinghong 149 100 1840 602 107 12.33 2.49 1500 833 806 Apr. 2008 Total 411.87 230.83 14800 6999 6929 the six dam reservoirs have an accumulated total and active reservoir reservoir filling behind the Dachaoshan, Jinghong and Xiaowan dams storage capacity of approximately 412 × 108 m3 and 231 × 108 m3,re- in combination with the severest drought for many years (Qiu, 2010) spectively, accounting for 48.9% and 27.4%, respectively, of the annual resulted in the decline of monthly mean flows except in April and Octo- mean discharge at Chiang Saen (842 × 108 m3 per year from 1960 to ber and subsequently produced a new intra-annual flow distribution 2009 (Lu et al., 2014)), which is the nearest gauging station down- pattern. Following the closure of the Xiaowan Dam, the monthly stream of the Chinese dams. mean flow further decreased during the high-flow months from June to October and slightly increased in most of the low-flow months. 3. Post-dam environmental consequences The latest findings also suggest that the Lancang cascade has likely led to a weak decline in the annual mean discharge at Chiang Saen 3.1. Hydrological effects from 858 × 108 m3 during the pre-dam period (1960–1991) to 815 × 108 m3 during the post-dam period (1992–2010) (Lu et al., Dams and their adjacent reservoirs significantly affect a river's hy- 2014). Moreover, the largest existing dam, Nuozhadu, began filling in drology, mainly through alterations of the five critical components of November 2011, and it is likely to augment the alteration of the river flow regime: magnitude, frequency, duration, timing, and rate of flow regime below the dam. To mitigate the hydrological impacts of change; ultimately, such changes yield a post-dam hydrologic regime the upstream dams, the Ganlanba Dam has been designed to anti- that is distinct from the natural flow regime (Poff et al., 1997; regulate the flows downstream, and construction is planned to begin Magilligan and Nislow, 2005). Generally, dam operations shift water in 2015. from the wet to the dry season via reservoir storage and result in lower average maximum flow, less interannual variation in daily maxi- 3.2. Upstream reservoir siltation and downstream sediment discharge mum and minimum flows, and higher minimum flow magnitudes (Poff decline et al., 2007). According to Vörösmarty et al. (2003), large dams intercept more than 40% of the water discharge of rivers globally. Dams disrupt the downstream transport of sediment, and at present, Over 75% of the outflowing annual mean water discharge from the they represent the greatest influence on land–ocean sediment fluxes. Lancang River of approximately 640 × 108 m3 occurs during the high- Worldwide dams are estimated to intercept approximately 4–5Gtor flow months from June to October, and the total represents only 13.5% 25–30% of the total annual sediment, with an average trapping efficien- of the total Lancang-Mekong discharge (4750 × 108 m3) that enters cy of more than 50% (Vörösmarty et al., 2003). The Lancang River de- the South China Sea (Zhao et al., 2000; Adamson et al., 2009). However, livers heavy sediment loads from the upper reaches of its watershed a combination of the upstream sections, which are mainly replenished through the Lower Mekong River to the South China Sea, and it contrib- by the glaciers and snowfields of the Qinghai–, and the utes approximately 50% of the total Mekong-borne sediment flux of rainy season in spring provides a large fraction of the downstream about 160 million tons annually to the sea (Milliman and Syvitski, flow in the dry season (Guo, 1985; He and Zhang, 2005). From April to 1992; Walling, 2009). There is no doubt that the construction and oper- June, the watershed areas above the Jiuzhou gauging station contribute ation of the Lancang cascade dams have trapped a portion of the incom- over 75% of the flow at the Yunjinghong gauging station, with the low- ing sediment load from the upstream river basin and subsequently est flow in March (Guo, 1985). Farther downstream at Kratie located in altered the sediment regimes of the Lancang-Mekong River. The the Lower Mekong River, the upper sections of the Lancang River also estimated potential trapping efficiency of the entire cascade of the provide approximately 30% of the lowest flow during April (Adamson eight dam reservoirs in the middle and lower Lancang River may et al., 2009). Therefore, the occurrences of abnormally low-dry season reach 78–81% (Kummu et al., 2010). Among these reservoirs, Xiaowan flows in the Lower Mekong River in the 1990s and 2000s triggered do- and Nuozhadu have a large potential trapping efficiency of up to 90%, mestic and international debates regarding the magnitude and extent of whereas Manwan, Dachaoshan, and Jinghong have a moderate trapping the hydrological impacts of Lancang's mainstream dams (Table 2). More efficiency of approximately 50%, which is smaller than the estimated recent studies indicated that there was no observed evidence that up- value proposed by Fu and He (2007) and Kummu and Varis (2007) of stream reservoir storage or release in China considerably altered the over 60%. The multiyear bathymetric survey, which is conducted by flood hydrology (e.g., 2008) or low-flow hydrology (e.g., 1992–1993, the Hydrolancang in the Manwan Dam reservoir once every three to 1996–1997, and 2003–2004 dry seasons) and that the low water levels five years, has shown that the volume of deposited sediment below of the Lower Mekong River were mainly attributed to low rainfall and the normal water level of 994 m was 5.48 × 108 m3,and92.3% accelerating deforestation rather than the Chinese Yunnan cascade (i.e., 5.06 × 108 m3) of this sediment occurred below the dead water dams (Lu and Siew, 2006; Campbell, 2007; MRC, 2008; Adamson levelof982mfrom1993–2009; in addition, there was a 16.7% loss of ac- et al., 2009; Lu et al., 2014). However, this view had been challenged tive storage capacity, 75.9% loss of dead storage capacity, and by others (Sretthachau and Deetes, 2004). 41.1 × 106 t (a bulk density of approximately 1.2 t/m3) of sediment As illustrated in Fig. 2, the Manwan Dam exerts trivial influences trapped annually on average by the reservoir, which is equivalent to over the annual mean flow and its intra-annual variation at the 76.5% of the annual incoming sediment of 53.7 × 106 tfrom1983– Yunjinghong gauging station. Throughout the 2000s, successive 1992 (Fig. 3a,b). The actual trapping efficiency of the Manwan reservoir H. Fan et al. / Earth-Science Reviews 146 (2015) 77–91 81

Table 2 Existing studies on the hydrological impacts of the Lancang cascade dams on the Lancang-Mekong River.

Methodology Data/model Hydrological changes Source

Statistical analysis based Daily water discharge at Chiang Saen (1960–2010). The existing dams altered the water flow to a certain degree Lu et al. (2014) on observational data at Chiang Saen. The post-dam monthly mean water discharge was 15% higher in July but 9% lower in August compared with the pre-dam values. Water discharge in the dry seasons (1-, 3- and 7-day minima) was obviously lower in the post-dam period than in the pre-dam period, whereas water discharge in the wet seasons (1-, 3- and 7-day maxima) was marginally lower in the post-dam period. Monthly water discharge at the Jiuzhou, Gajiu, and There was an increase in maximum monthly water Zhao et al. (2012) Yunjinghong gauging stations (1957–2000). discharge in the post-dam period compared with the pre-dam period. Annual water discharge at Chiang Sean, Luang Prabang, No significant change in precipitation and runoff occurred Xue et al. (2011) Vien Tiane, Nong Khai, Nakhon Phanom, Mukdahan, over the past 50 years. The runoff presented a closer Khong Chiam, and Pakse (1960–2005); daily connection with the regional precipitation and ENSO maximum and minimum water levels at Can Tho and activity during the post-dam period. Both the daily My Thuan on the delta plain. maximum and minimum water levels on the delta plain have shown an abrupt drop since the end of 1994. Daily water discharge at Vientiane (1913–2000), Dams were not the cause for the altered behavior of the Delgado et al. (2010) Thakhek (1924–2000), Pakse (1923–2000) and Kratie flood variability during the last 20 years of the 20th century (1924–2007). at Vientiane and downstream stations. Monthly water levels at the Jiuzhou, Gajiu, and The effects of the dams on water levels are limited at the Li and He (2008) Yunjinghong gauging stations (1960–2003) annual mean and wet season mean levels and are not obvious at the monthly and yearly time scales. However, intra-annual variations of water levels at Gajiu and Yunjinghong were affected. Monthly water discharge at Chiang Saen (1961–2004), The flows tended to increase in the dry season Campbell (2007) Mukdahan (1950–2004) and Kratie; daily water (January–May) and decrease in the wet season discharge at Pakse (1924–2004). (June–November). A significant reduction in the monthly discharge occurred in August at Chiang Saen, in September and October at Mukdahan, and in October and November at Pakse. However, significant dry season increases occurred in March and April at Mukdahan and in March, April and May at Pakse. At Pakse, there was a significant increase in the annual minimum daily discharge but an insignificant decrease in the annual maximum daily flow. Annual water discharge at Chiang Saen, Luang Prabang, No impacts on the annual water discharge. However, there Kummu and Varis (2007) Nong Khai, Mukdahan and Pakse (1962–2002) was an increase in the mean flow at Luang Prabang and Pakse in the post-dam period (1993–2000) compared with the pre-dam period (1962–1992). Annual water discharge at Chiang Saen, Luang Prabang, No significant changes in the annual mean, maximum and Lu and Siew (2006) Vientiane, Nongkhai, Nakhon Phanom, Mukdahan, minimum discharge along the Lower Mekong River except Khong Chiam, and Pakse (1962–2000). at several stations. Following the operation of the Manwan Dam, the frequency and magnitude of the water level fluctuations increased considerably during the dry season. Hydrological modeling A combination of a hydrological model and a reservoir The Lancang cascade significantly altered the Mekong's Rasanen et al. (2012) cascade optimization model with a spatial resolution of hydrological regime; the December–May discharge 5 km × 5 km grid cells. increased by 90% and the June–November discharge decreased by 22% at Chiang Saen. A distributed hydrological model, VMod, with a spatial The operation of planned dams does not significantly impact Lauri et al. (2012) resolution of 5 km × 5 km grid cells. the annual water discharge, but it causes 25–160% higher dry season (December–May) discharges at Kratie, 41–108% higher dry season discharges at Chiang Saen, 5–24% lower wet season discharges at Kratie and 3–53% lower wet season discharges at Chiang Saen. The discharge decreases by 24% at Kratie and 53% at Chiang Saen during the beginning of the wet season in July. The discharge decreases by 8% at Kratie and 13% at Chiang Saen during September, the wettest month. Semi-distributed Land Use-based Runoff Processes The hydrology of the Mekong River Basin was mainly Kingston et al. (2011) (SLURP) with a spatial resolution of 0.5° × 0.5° grid impacted by climate change, which obviously increased the cells. discharge from April–June and decreased the discharge in July and August. For the Lancang sub-basin, the April–June discharge increase was temperature-driven. DSF simulation models, the SWAT hydrological model, Water resource development will cause a decrease in the Hoanh et al. (2010) the integrated quantity/quality model (IQQM) for overall annual discharge of approximately 3–8%. Conversely, basin simulations and the hydrodynamic ISIS model at climate change will increase the river discharge by 6–16%. a spatial resolution of approximately 22 km × 22 km Water resource development introduces a river discharge grid cells. increase of 30–60% during the low-flow season and a decrease of 8–17% during the high-flow season. Yamanashi Hydrological Model (YHyM) at a spatial No significant trend in the annual water discharge during Hapuarachchi et al. (2008) resolution of 2′ ×2′ grid cells. the period from 1980–2000.

is much higher than the previously estimated values (Fu and He, 2007; of a dam. Nevertheless, with the construction of the much larger Fu et al., 2008b; Kummu et al., 2010), and severe reservoir siltation Xiaowan Dam upstream of the Manwan Dam and implementation of poses a great threat to the annual generation and operational lifespan the Grain for Green Program (or Sloping Land Conversion Program) in 82 H. Fan et al. / Earth-Science Reviews 146 (2015) 77–91

4000 Simulated natural flow without dam (1994-2001) 3500 Observed streamflow after the closure of Manwan dam (1994-2001)

/s) 3000 3 Simulated natural flow without dams (2010) 2500 Observed streamflow after the closure of Xiaowan dam (2010) 2000

1500

1000 Monthly mean flow (m

500

0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Fig. 2. Dam-induced alterations in monthly streamflow at the Yunjinghong gauging station. Source: KHIDI, 2012. the Lancang River Basin, the average annual sedimentation sharply concentrations along the middle and lower Mekong River have shown dropped from 0.31 × 108 m3 (2003–2005) to 0.12 × 108 m3 (2005– that the trends in sediment load at the Chiang Saen gauging station 2009) (Fig. 3b). Thus, the long-term operation of the Manwan Dam and downstream mainstreams are rarely consistent (Lu and Siew, will improve in the future. 2006; Kummu and Varis, 2007; Liu et al., 2013). Wang et al. (2011) re- Large downstream sediment contributions and high sediment trap- constructed annual sediment load records from 1962 to 2003 for major ping efficiencies have resulted in increasing concern for the potential gauging stations along the Mekong River (i.e., Chiang Saen, Luang downstream sediment impacts of dam construction. However, the Prabang, Nong Khai, Mukdahan and Khong Chiam) using water dis- existing literature has presented conflicting findings regarding the na- charge and sediment concentration data from different sources; they re- ture and magnitude of recent changes in the sediment load on the ported that at the Chiang Saen gauging station, the sediment load Lancang-Mekong River (Liu et al., 2013). A significant reduction in the during the post-dam period (1993–2003) was approximately 30% mean annual sediment load has been reported by certain researchers lower than that during the previous period (1986–1992) and approxi- (Lu and Siew, 2006; Kummu and Varis, 2007; Fu et al., 2008a,b); howev- mately 15% higher than that during the more previous period (1962– er, Walling (2008, 2009) suggested that the construction of the Manwan 1985). Because existing studies have failed to distinguish the impact dam exerted little impact on the mean annual sediment load along the of human activities from the impact of climate variations and hydrolog- middle and lower Mekong River between the 1960s and 2000s because ical cycles and have not considered the additional incoming sediment of the continued increase of sediment loads from the late 1960s to mid- during the construction period of the dam, attributing the fluctuations 1990s in the Lancang River that are associated with increasing human of sediment loads to dam construction is premature (Wang et al., activities in the middle and lower reaches of this basin (You, 1999). 2011). However, it appears inevitable that the progressive completion The drastic decline in the mean annual sediment load has only appeared of the cascade of dams on the Lancang River will significantly reduce in the river section upstream of the Yunjinghong gauging station, the incoming sediment to the downstream mainstream because of the whereas farther downstream, less evidence has been found of a de- expansion of controlling watershed areas above these dams and in- crease in sediment load (Fu et al., 2008b; Liu et al., 2013). The sporadic crease of the long-term trapping efficiency along the cascading reser- and unrepresentative observational data on suspended sediment voirs. If additional regional sediment yields from the downstream

a) b) 6 0.6 ) 3 ) 3 m 0.5 8

m 5 8

0.4 4

0.3 3

0.2 2

0.1

1 Volumn of deposited sediment (×10 Volumn of deposited sediment (×10

0 0 1996 2000 2003 2005 2009

1996 2000 2003 2005 2009 1993- 1996- 2000- 2003- 2005-

Fig. 3. Accumulated total (a) and average annual (b) sedimentation in the Manwan dam reservoir after dam impoundment. Source: Zhao et al., 2013. H. Fan et al. / Earth-Science Reviews 146 (2015) 77–91 83 watershed do not occur, then the sediment load of the Mekong River also suggested that there was no evidence other than in the Mekong can be expected to greatly decrease (Kummu et al., 2010). Although re- River Delta that the water quality was continually exacerbated. duced sediment loads can facilitate irrigation and navigation on the Me- Upstream from the dam, post-dam heavy metal concentrations in kong River, they are likely to reduce the nutrient fluxes in the river and the water were more spatially heterogeneous and less temporally vari- cause erosion of the downstream riverbanks and Mekong Delta. To fully able except for those in the dam reservoir (Yao et al., 2005). As for the assess the downstream cascade impacts of the dams on sediment fluxes, reservoir waters, the concentrations of Cu and Cd tended to decline, an expanded, intensified and integrated transboundary sediment- whereas the Pb concentration increased then decreased and Zn concen- monitoring program that includes measurements of physical and geo- tration fluctuated at the initial stage of reservoir filling and then tended + chemical properties (e.g. loss on ignition, particle size, and trace metal to increase (Fig. 4). The concentrations of NH4 –N and total phosphorus concentration and behavior) of suspended sediment is urgently re- (TP) in the reservoir waters also exhibited different temporal patterns + quired. The current monitoring is oriented towards collecting data to (Fig. 4). The NH4 –N concentration tended to increase (1993–1997), de- calibrate and validate existing models instead of performing a tradition- crease (1997–2001), and then increase again (2001–2004), whereas the al statistical analysis of the frequency of runoff and sediment transport. TP concentration has consistently increased from 0.032 mg/L to 0.1 mg/L since 1997. In addition, the intra-annual variations of heavy metal and nutrient concentrations in the reservoir waters also fluctuat- 3.3. Changes in water quality ed as a result of the operation of the dam (Fig. 5). The Pb concentration increased in the normal-flow season, decreased in the low-flow season, Dam impoundment usually exerts bidirectional effects on the quali- and decreased slightly within a narrow range during the high-flow pe- ty of water (Baxter, 1977). Recently, additional attention has been fo- riod. The Cu concentration increased in the low-flow season, decreased cused on the pollution of water resources and the impacts on human in the high-flow season, and decreased slightly within a narrow range + water security and biodiversity as a result of increasing eutrophication during the normal-flow period. The post-dam concentrations of NH4 – in the reservoir areas (Vörösmarty et al., 2010; K.F. Li et al., 2013; Xu N and TP had similar tendencies of higher values in the low-flow and et al., 2013). The water quality of the mainstream Lancang River normal-flow seasons and smaller values in the high-flow season. is often rated as grade II or III (e.g., grades I, II, and III are suitable for Water temperature is another important water quality parameter. drinking) because fewer pollution sources are associated with the The complex flow pattern in large high-dam reservoirs can exert an im- less-developed regional economy in the basin (CLSCYU and MHSYP, portant effect on the downstream temperature regime (Baxter, 1977). 2000). Subsequent to the reservoir impoundment, the transformation The overall effect stratifies the water in the reservoir; thus, the outflow of lotic and floodplain habitats into lentic habitats slowed the flow ve- below the dam is cooler in summer and warmer in winter than it was locity of the Lancang River, and the self-purification capacity declined. before the dam closure. After the closure of the Manwan dam, the Consequently, the quality of water in the reservoirs has changed monthly temperature of the reservoir surface water greatly increased (CLSCYU and MHSYP, 2000; Yao et al., 2005; Zhang et al., 2005). (Fig. 6). In contrast, the water temperature of the Gajiu gauging station A 15-year monitoring study conducted by the Yunnan Province Cen- downstream of the dam declined in the months from March to May and tral Environmental Monitoring Station indicated that since the commis- increased to different degrees in the remaining months. The low- sion of the Manwan Dam, the water quality in the mainstream Lancang temperature water released by the dam reservoir can undoubtedly af- River downstream from the dam had no obvious change and actually fect fish distributions and the downstream behavior of the dam, with improved in the river section downstream from the dam to the migratory fish the most affected. Freshwater fish provide a large fraction Yunjinghong gauging station (Zhang et al., 2005). As a result of the sed- of animal protein sources for local residents in the downstream riparian iment deposit in the dam reservoir, a sharp downstream decrease in the countries and are an important focus of the potential downstream im- concentrations of heavy metal pollutants and investigated nutrients pacts of the Lancang cascade dams. With the impoundment of the two + (NH4 –N, MnO4-andPO4-P) occurred since the closure of the dam. Al- multi-year regulating storage reservoirs, i.e., Xiaowan and Nuozhadu, though the water quality in some river sections below the Yunjinghong the low-temperature outflow from both reservoirs will further cool gauging station deteriorated to grade IV or to grade V in some cases in the downstream water during the dry season. To restore the down- 2000 as a result of sugar and rubber factories and fertilizer usage in stream water temperature to the pre-dam level, a multi-level intake Xishuangbanna Prefecture, the water quality of the outflow from structure with a stop-log gate will be adopted in the Nuozhadu Dam China was ranked as grade III (Zhang et al., 2005). Campbell (2007) (Gao et al., 2012), and its effects will be monitored and verified.

a) b) 0.16 3.5 0.45 0.12 + Cu NH4+-NNH4 -N 0.14 0.40 TP Cd 3 0.1

mg/L) 0.35 0.12 Pb 2 2.5 Zn 0.30 0.08 0.10 2 0.25 0.08 0.06 1.5 0.20 0.06

-N concentration (mg/L) 0.15 0.04 + TP concentration (mg/L)

1 4

Cu concentration (mg/L) 0.04 0.10 NH 0.5 0.02 0.02 Pb/Zn/Cd concentration ( × 10 0.05

0.00 0 0.00 0 1987 1988 1989 1990 1991 1993 1995 1997 1999 2001 2003 2004 1989 1990 1991 1993 1995 1997 1999 2001 2003 2004

Fig. 4. Interannual variation in concentrations of heavy metals (a) and nutrients (b) in the Manwan dam reservoir waters. Adapted from Yao et al., 2005. 84 H. Fan et al. / Earth-Science Reviews 146 (2015) 77–91 a) b) c) d)

0.16 Pre-dam 0.02 Pre-dam 0.40 Pre-dam 0.03 Pre-dam Post-dam Post-dam Post-dam Post-dam 0.14 0.01 0.35 0.03 0.12 0.01 0.30 0.02 0.10 0.01 0.25

0.08 0.01 0.20 0.02

0.06 0.01 0.15

-N Concentration (mg/L) 0.01 + 4 Pb Concentration (mg/L) Cu Concentration (mg/L) 0.04 0.00 0.10 TP Concentration (mg/L) NH 0.01 0.02 0.00 0.05

0.00 0.00 0.00 0.00 LFP HFP NFP LFP HFP NFP LFP HFP NFP LFP HFP NFP

Fig. 5. Intra-annual variation in concentrations of heavy metals (a, b) and nutrients (c, d) in the Manwan dam reservoir waters. LFP: low-flow period; HFP: high-flow period; and NFP: normal-flow period. Adapted from Yao et al., 2005.

4. Ecological consequences of dam-triggered gases (Fig. 7a). Accordingly, the abundance of phytoplankton assem- environmental changes blages increased from 1.10 × 105 ind./L to 4.92 × 105 ind./L in the tran- sitional zone and from 1.06 × 105 ind./L to 13.77 × 105 ind./L in the 4.1. Responses of phytoplankton and zooplankton lentic zone, whereas the number dropped from 1.44 × 105 ind./L to 0.55 × 105 ind./L in the fluvialzonebelowthedam(Fig. 7b). Subsequent Phytoplankton and zooplankton can respond quickly to environ- to the reservoir impoundment, Bacillariophyta still dominated the phy- mental changes and can be regarded as effective indicators for monitor- toplankton assemblages, but the total number of species greatly de- ing subtle alterations in water quality (Webber et al., 2005; Jeppesen creased, especially in the fluvial zone below the dam. In contrast, both et al., 2011; Thackeray et al., 2013). Following the rapid transformation the total number of species and abundance remarkably increased for of a lotic stream ecosystem to a lentic ecosystem in the inundated river Chlorophyta and slightly increased for other phytoplankton, whereas sections above a dam, impoundment may affect the plankton popula- some species belonging to Euglenophyta, Pyrrophyta, Cryptophyta, tion in the dam reservoir in a variety of ways that are mainly a result and Xanthophyta were newly observed after the closure of the dam. of the reduction of flow rate and addition of nutrients from leached Subsequent to the construction and operation of the upstream Xiaowan soil and decomposed drowned vegetation (Baxter, 1977). Regarding Dam, the phytoplankton assemblages rapidly responded to further en- the Manwan Dam reservoir, pre- and post-dam in situ monitoring vironmental change in the Manwan reservoir and downstream areas. data have shown that reservoir impoundment clearly affects the com- During the construction period of the Xiaowan dam, the abundance of position, abundance, and biomass of phytoplankton and zooplankton phytoplankton assemblages sharply dropped from 1.54 × 105 ind./L to assemblages (Figs. 7 and 8). After the Manwan Dam was commissioned, 0.05 × 105 ind./L in the tail fluvial zone of the Manwan reservoir and the total number of phytoplankton species increased from 52 to 99 in from 9.34 × 105 ind./L to 0.10 × 105 ind./L in the lentic zone of the res- the central transitional zone of the reservoir and from 43 to 99 in the ervoir. In contrast, the abundance significantly increased from lentic zone near the dam. In contrast, the number decreased from 41 0.55 × 105 ind./L to 1.18 × 105 ind./L in the fluvial zone below the to 35 in the fluvial zone below the dam, which might have been a result Manwan Dam (i.e., the tail fluvial zone of the Dachaoshan reservoir in of the release of low-temperature water that was supersaturated with 2003) (Fig. 7c).

a) b) 26 26

24 24

C) 22 22 ˚ C) ˚ 20 20

18 18

Temperature ( 16 16 Temperature (

14 Pre-dam 14 Pre-dam Post-dam 12 Post-dam 12

10 10 Jul Jul Jan Jan Jun Jun Oct Oct Sep Feb Feb Sep Apr Apr Dec Dec Mar Mar Aug Nov Aug Nov May May

Fig. 6. Pre- and post-dam water temperatures at the Manwan Dam reservoir (a) and Gajiu gauging station (b). Source: CLSCYU and MHSYP, 2000; Zhang, 2001. H. Fan et al. / Earth-Science Reviews 146 (2015) 77–91 85 a) Fluvial zone Transitional zone Lentic zone Below the dam 120 Bacillariophyta

Rhodophyta 100 Xanthophyta

80 Cryptophyta

Pyrrophyta 60 Chrysophyta

Number of species Euglenophyta 40 Cyanophyta

20 Chlorophyta

0 1984 1997-1998 1984 1997-1998 1984 1997-1998 1984 1997-1998

b) Fluvial zone Transitional zone Lentic zone Below the dam 16

14 Pyrrophyta

ind./L) Chrysophyta

5 12 Euglenophyta 10 Cyanophyta Chlorophyta 8 Bacillariophyta 6

4

2 Abundance of species (×10

0 1984 1997-1998 1984 1997-1998 1984 1997-1998 1984 1997-1998

c) Fluvial zone Transitional and Lentic Below the dam 10 Rhodophyta zones 9 Xanthophyta

8 Cryptophyta ind./L) 5 7 Pyrrophyta

6 Chrysophyta 5 Euglenophyta 4 Cyanophyta 3 Chlorophyta 2 Bacillariophyta 1 Abundance of species ( × 10 0 1997-1998 2003 2011 1997-1998 2003 2011 1997-1998 2003 2011

Fig. 7. Pre- and post-dam species number and composition (a), and species composition and abundance (b,c) of phytoplankton in the Manwan dam reservoir. Source: CLSCYU and MHSYP, 2000; Zhang, 2001; IHE, 2012.

Upon the closure of the Xiaowan Dam, the abundance of the phyto- The zooplankton assemblages changed in species number, composi- plankton assemblages in the above three reservoir zones drastically in- tion, abundance and biomass following the impoundment of the creased to 7.86 × 105 ind./L in the tail fluvial zone of the Manwan Manwan Dam (Fig. 8). The species number of zooplankton significantly reservoir, 8.65 × 105 ind./L in the lentic zone of the reservoir and increased from 45 in 1984 to 148 in 1994, 142 in 1996 and 133 in 1997 3.00 × 105 ind./L in the fluvial zone below the Manwan dam (Fig. 7c). (Fig. 8a). Accordingly, the individual density of zooplankton also in- In addition, the proportion of Bacillariophyta greatly decreased, where- creased from 1035 ind./L in 1984 to 2547 ind./L in 1994, 1799 ind./L in as the proportion of Chlorophyta and other phytoplankton clearly in- 1996 and 3791 ind./L in 1997 (Fig. 8b). The proportions of Cladocera creased in the reservoir areas during the construction of the Xiaowan and Copepoda species greatly increased, whereas rotifers species de- Dam. The rapidly increasing abundance and biomass of phytoplankton creased and the change in protozoa species was smaller. The dynamics assemblages in the dam reservoir areas can certainly lead to degrada- of zooplankton in the reservoir varied considerably according to season tion of the aquatic ecosystem (J. Li et al., 2013a), and water blooms of and reservoir zone. As shown in Fig. 8b, the individual density and bio- Dinophyceae reportedly occurred in the reservoir section 30 km above mass of zooplankton in February 1998 was twice that of February 1984 the Manwan Dam in February 2003 (Wang et al., 2004). for the lentic zone of the reservoir near the dam. For the central 86 H. Fan et al. / Earth-Science Reviews 146 (2015) 77–91 a) 160

140 Other zooplankton 120 Copepoda 100 Cladocera Rotifers 80 Protozoa 60 Number of species 40

20

0 1984 1994 1996 1997 b) 3000 2.5 Abundance in the fluvial zone 2500 2 Abundance in the transitional zone 2000 1.5 Abundance in the lentic zone 1500 Biomass 1 Biomass (mg/L)

Abundance (ind./L) 1000

0.5 500

0 0 Feb. 1984 Feb. 1998 Jul. 1997 Jul. 2011

Fig. 8. Pre- and post-dam species number and composition (a), abundance and biomass (b) of zooplankton in the Manwan dam reservoir. Source: CLSCYU and MHSYP, 2000; Zhang, 2001; IHE, 2012.

transitional zone of the reservoir, the individual density of zooplankton affect the security of the aquatic ecosystem in the middle and lower in February 1998 was three times that of February 1984, whereas the reaches of the Lancang River. Thus, effective protection measures along pre- and post-dam biomasses of zooplankton were roughly equivalent. with environmental monitoring should be implemented or prepared so After the closure of the Xiaowan Dam, the individual densities of zoo- that the health of the aquatic ecosystem may be sustained. plankton decreased by 98 ind./L and 1916 ind./L in the tail fluvial and lentic zones, respectively, of the reservoir in July 2011 compared to 4.2. Responses of the zoobenthos the values in July 1997. In addition, the biomass of zooplankton sharply dropped in the lentic zone and slightly increased in the fluvial zone. In Benthic biological communities play a crucial role in the functioning the transitional zone, changes in the individual density and biomass of of the river ecosystem and are often indicative of the changing water zooplankton were small. A recent study showed that the densities of environment. There are 59 species of benthic invertebrates recorded zooplankton communities in the Xiaowan Dam reservoir were positive- in the middle reach of the Lancang River from 1988 to 1998. They in- ly related to temperature, total phosphorus, turbidity, and Chl-a, where- clude 4 species and 4 genera in 2 families of Annelida, 6 species and as they were negatively related to total nitrogen and conductivity (Wu 5 genera in 3 families of Mollusca, and 49 species and 42 genera in 30 et al., 2014). Therefore, it can be deduced that the reduction in the indi- families of Arthropoda (CLSCYU and MHSYP, 2000). Before filling the vidual density and biomass of zooplankton is attributable to the opera- reservoir, the benthic fauna was sparse in the river section of the tion of the Xiaowan Dam. Manwan Dam reservoir, which was dominated by zoobenthos species A recent ecological risk assessment of hydropower dam construction of flowing water taxa, such as the larva of Ephemerida and Trichoptera on phytoplankton and zooplankton species based on the ESHIPPO model (CLSCYU and MHSYP, 2000). Subsequent to the reservoir impound- showed that endemic species, including Bangia atropurpurea (Roth.) ment, the benthic population was more diverse and larva of Chironomid Ag., Lemanea (Sacheria) sinica, Prasiola sp., Attheyella (Mrazekiella) and water earthworms became dominant (CLSCYU and MHSYP, 2000). yunnanensis,andNeutrodiaptomus mariadviagae (Brohm), were at high The total number of zoobenthos species increased from 2 in 1994 to 10 ecological risk, whereas the widely distributed species of phytoplankton in 2011 (Fig. 9a). The more drastic increase occurred in the transitional and zooplankton were at medium ecological risk in the middle reaches of and lentic zones, where the number of species increased individually the Lancang River (X. Li et al., 2013). With the successive construction from 1 in 1994 to 8 in 2011, due to a change from a lotic to a quasi- and operation of the Lancang 21-dam cascade over a considerable lentic or lentic system (Fig. 9b). Accordingly, species composition range of latitudes, homogenization of phytoplankton and zooplankton changed greatly. Before 1997, only species of Arthropoda was collected faunas will inevitably occur along the series of reservoirs because north- in the Manwan Dam reservoir and the number of species increased over ern species will be introduced to more southerly regions (Moyle and timefrom2in1994to10in1997(Fig. 9b). In 1998, one fouling species Mount, 2007), which will pose a threat to southern species and further of Annelida, such as Tubifex sinicus, and 4 species of Arthropoda were H. Fan et al. / Earth-Science Reviews 146 (2015) 77–91 87 a) 12

10

8

6

4 Number of species

2

0 1994 1996 1997 1998 2011

b) Fluvial zone Transitional zone Lentic zone 9

8 Arthropoda 7 Mollusca 6 Annelida

5

4

Number of species 3

2

1

0 1994 1996 1997 1998 2011 1994 1996 1997 1998 2011 1994 1996 1997 1998 2011

c) Fluvial zone Transitional zone Lentic zone 600 20

Abundance 18 500 16 ) 2 Biomass

14 ) 400 2 12 300 10 8 200 Biomass(g/m

Abundance (ind./m 6 4 100 2 0 0 1996 1997 1998 2011 1996 1997 1998 2011 1996 1997 1998 2011

Fig. 9. Post-dam dynamics in species number (a), composition (b), abundance and biomass (c) of zoobenthos in the Manwan dam reservoir. Source: CLSCYU and MHSYP, 2000; Zhang, 2001; IHE, 2012.

individually found in the transitional and lentic zones, and only one spe- greatly increased from the tail fluvial zone through the transitional cies of Arthropoda occurred in the fluvial zone. After the commission of zone and lentic zone and positively varied with the operation time of the Xiaowan Dam, 2 species of Annelida, 2 species of Mollusca and the reservoir (Fig. 9c). After the operation of the Xiaowan Dam, howev- 6 species of Arthropoda were collected in 2011. Species of Mollusca er, this downstream increase did not occur. The highest individual den- and Arthropoda were simultaneously observed in the three zones of sity and biomass of zoobenthos occurred in the central transitional zone the reservoir, whereas species of Annelida only occurred in the lentic of the Manwan Dam reservoir and lowest occurred in the tail fluvial zone above the dam. zone of the reservoir. Accordingly, the individual density and biomass The post-dam individual density and biomass of zoobenthos species of zoobenthos in the transitional zone significantly increased from reportedly increased and that their values in 1998 were 4.3 times and 217 ind./m2 and 7.63 g/m2, respectively, in 1998 to 520 ind./m2 and 98.8 times, respectively, those in 1988 (CLSCYU and MHSYP, 2000). 18.80 g/m2, respectively, in 2011, whereas these values decreased in Along the river, the individual density and biomass of zoobenthos the lentic zone from 399 ind./m2 and 14.79 g/m2, respectively, in 1998 88 H. Fan et al. / Earth-Science Reviews 146 (2015) 77–91 to 201 ind./m2 and 9.55 g/m2, respectively, in 2011 (Fig. 9c). Regarding river damming is the most serious threat to fish in the mainstream the tail fluvial zone of the reservoir, the individual density of Lancang River; over-fishing and the introduction of exotic species are zoobenthos sharply dropped, whereas the biomass inversely increased secondary threats (Kang et al., 2009). from 0.13 g/m2 in 1998 to 0.27 g/m2 in 2011. The impact of dam construction on fish species varied considerably according to the specific river sections (Fig. 10). Subsequent to river 4.3. Responses of fish resources damming, the number of aboriginal fish species decreased by various degrees in the dam reservoirs of Manwan, Jinghong and Xiaowan but The effects of river damming on fish species have caused increasing not in the Dachaoshan Dam reservoir. After the closure of the concern among scholars in related fields worldwide as a result of the in- Dachaoshan Dam, the number of aboriginal fish species in the reservoir undation of habitats, isolation of fish populations, and interruption of increased from 10 in 2002 to 25 in 2011, which was likely related to the migratory paths triggered by dam blockage and reservoir impoundment fewer sampling sites in 2002. Downstream of the Jinghong Dam, there (Park et al., 2003; Ziv et al., 2012). According to extant records, the mid- was no observed evidence that the aboriginal fish species had been dle and lower Lancang River provides habitat for a total of 165 species of negatively affected as a result of the interruption of their migratory pas- fish that are distributed among 11 orders, 28 families and 100 genera sages or loss of feeding and spawning grounds. Two major less dis- (Zheng et al., 2013). Of these species, 9 are endangered species listed turbed (i.e., the Buyuan River and Nanla River) below the in the China Red Data Book of Endangered Animals: Gyrinocheilus Jinghong Dam were identified as important feeding and spawning hab- aymonieri, Macrochirichthys macrochirius, Pangasius sanitwongsei, itats and fish passages for upstream migrant species (Kang et al., 2009). Hampala macrolepidota, Cosmochilus cardinalis, Puntioplites proctozysron Damming both tributaries could have a greater negative effect on fish (Puntioplites falcifer), Kryptopterus moorei, Akysis brachybarbatus, and biodiversity than damming the main river (Ziv et al., 2012); thus, Bagarius bagarius. However, only 71 of these species have been collected these areas should have the highest priority for conservation. on field visits during 2008–2012 (Zheng et al., 2013); the species are In addition to the reduction in the number of aboriginal fish species, distributed in 6 orders, 12 families and 52 genera. Among these, reservoir impoundment also altered the spatial distribution patterns of Cypriniformes, Siluriformes and Perciformes are the most species-rich fish species in the middle and lower reaches of the Lancang River. Before orders in the middle and lower Lancang River. Along the mainstream river damming, there were 17 lotic fish species, 28 running-water fish Lancang River, certain fish species with large individuals, such as Tor species, and 7 lentic fish species randomly distributed along the river Gray, Bagarius and Bangana, have greatly decreased in relative abun- channel of the Manwan Dam reservoir (CLSCYU and MHSYP, 2000). dance, whereas the relative abundance of fish species with small to Upon dam impoundment, the lotic fish species tended to prefer the flu- medium individuals, such as Clupisoma longianalis, Mystacoleucus vial zone with the fast-flowing lotic habitat, whereas the running-water marginatus, Garra,andSchistura, have increased considerably and fish species preferred the transition zone and running-water habitat accounted for over 80% of the total fish catch (Liu et al., 2011; Zheng and lentic fish species preferred the lentic zone. The exotic fish species et al., 2013). In addition to H. macrolepidota and B. bagarius, other en- were mostly observed in the lentic zone near the dam. Similar results dangered species had not been harvested for many years. It is also re- were found at other existing dam reservoirs in the middle and lower ported that 12 of the aboriginal fish species were at high ecological mainstream Lancang River (J. Li et al., 2013b). After the closure of the risk (X. Li et al., 2013). Therefore, compared with the historical data, Xiaowan Dam, the Manwan Dam reservoir became a slow running- the current species numbers of aboriginal fish in the middle and lower water and lentic habitat, which resulted in a significant decline in the mainstream Lancang River, especially medium-large and rare-endemic number of lotic aboriginal fish species and large increase in the number species, have obviously decreased (Liuetal.,2011;Zhengetal.,2013). of exotic fish species. It is likely that the same change will occur in the Conversely, the species number of exotic fish increased from 10 in the Jinghong Dam reservoir when the Nuozhadu Dam becomes fully pre-dam period to 20 in 2011. The total number of aboriginal fish spe- operational. cies is considered a measure of the biological diversity, which generally To mitigate the above-mentioned negative effects of the construc- decreases with increasing environmental degradation (Roset et al., tion of the Lancang cascade dams on fish assemblages, a series of mea- 2007). Thus, the decline in fish species and resources might have been sures, including the identification of critical major tributaries for the caused by the large-scale dam construction, over-fishing, pollution establishment of nature reserves, conservation of habitat, removal and exotic fish. The alteration of hydrological regimes triggered by of dams on major tributaries, stock enhancement of aboriginal fish,

Below Jinghong nawnaM Dachaoshan awoaiXgnohgniJ n dam 60

50

40

30

20

10 Number of aboriginal fishspecies 0 1984 2011 2002 2011 2007 2011 2008 2011 1994-1998 2001-2002 2010-2011

Fig. 10. Pre- and post-dam species number of aboriginal fish in the major dam reservoirs along the middle and lower reaches of the Lancang River. Source: CLSCYU and MHSYP, 2000; Zheng et al., 2013. H. Fan et al. / Earth-Science Reviews 146 (2015) 77–91 89 reoperation of ecological reservoirs, establishment of improved eco- dams can be used as constructive references for sustainable hydropow- environment monitoring and evaluation systems, and administrative er development on other international rivers in southwestern China, in- management of fisheries, have been implemented (Yuan, 2012; cluding the Nu-, Yuan-, Dulong-Irrawaddy Huang, 2013). In addition, appropriate fish passage facilities, including River, and Yarlung Zangbo-Brahmaputra River. fish elevators, fishlocks, and fish ladders, were considered for the under- way and for planned dams along the mainstream Lancang River (Huang, 2013). However, a quantitative evaluation of the overall efficiency of the Acknowledgments above-mentioned measures in restoring fish passages and habitats is ur- gently required. We are grateful to two anonymous reviewers for the helpful sugges- tions. This study was financially supported by the National Science and chnology Support Program (2013BAB06B03), Natural Science 5. Concluding remarks Te Foundation of China (U1202232, 41461017, 41061010), China Huaneng Group Science & Technology Program (HNKJ13-H17-03), and The high gradient of physical, chemical and biological processes along the mainstream Lancang-Mekong River provides a variety of hab- Candidates of the Young and Middle Aged Academic Leaders of Yunnan Province (2014HB005). itats that support some of the world's most diverse terrestrial and aquatic communities as well as a large number of critically endangered, rare and endemic species. Large-scale hydropower cascade damming on References rivers, which is occurring at an unprecedented rate to satisfy the over- arching goal of increasing China's economic growth, entails significant Adamson, P.T., Rutherfurd, I.D., Peel, M.C., Conlan, I.A., 2009. Chapter 4 — The hydrology of the Mekong River. In: Campbell, I.C. (Ed.), The Mekong-Biophysical Environment of complexity and uncertainty regarding the scale and scope of potential an International River Basin. Academic Press, San Diego, pp. 53–76. http://dx.doi. environmental and ecological impacts. This paper has reviewed the org/10.1016/B978-0-12-374026-7.00004-8. major environmental consequences of the cascading dams along the Baxter, R.M., 1977. Environmental effects of dams and impoundments. Annu. Rev. Ecol. Syst. 8, 255–283. http://dx.doi.org/10.1146/annurev.es.08.110177.001351. mainstream Lancang-Mekong River. The existing literature and obser- Campbell, I.C., 2007. Perceptions, data, and river management: lessons from the Mekong vational data demonstrate that the cascading dams have led to a decline River. Water Resour. Res. 43 (2), W02407. http://dx.doi.org/10.1029/2006WR005130. in the flood season water discharge and annual sediment flux within Campbell, I., 2009. Chapter 1 — Introduction. In: Campbell, I.C. (Ed.), The Mekong- China's borders, reservoir aggradations, and degradation of water qual- Biophysical Environment of an International River Basin. Academic Press, San Diego, pp. 1–11. http://dx.doi.org/10.1016/B978-0-12-374026-7.00001-2. ity within the reservoirs. Furthermore, the dams have negatively affect- CLSCYU (The College of Life Sciences and Chemistry of Yunnan University), MHSYP ed the riverine aquatic biological communities and fish assemblages. In (Manwan Hydropower Station of Yunnan Province), 2000. The Ecological Environ- contrast, China's Lancang cascade dams have only produced minimal ment and Biological Resources of Manwan Hydropower Station Reservoir along Lancang River in Yunnan, China. Yunnan Science & Technology Press, Kunming (In unfavorable effects on the downstream environment outside of China. Chinese with English summary). However, the combined operation of the two multi-year regulating CNEA, CNREC, 2012. Key information at a glance China 12th Five-Year Plan for Renewable storage reservoirs, i.e., Xiaowan and Nuozhadu, is likely to produce Energy Development (2011–2015). National Energy Administration and China National Renewable Energy Center, Beijing, China (doi:http://en.cnrec.info/go/ stronger effects on the downstream environment and communities AttachmentDownload.aspx?id={2d5f4dfb-8d11-42de-832f-2bd1f933b493}). along the mainstream Lancang-Mekong River if appropriate and effec- Delgado, J.M., Apel, H., Merz, B., 2010. Flood trends and variability in the Mekong river. tive measures are not actively implemented. Moreover, the progressive Hydrol. Earth Syst. Sci. 14 (3), 407–418. http://dx.doi.org/10.5194/hess-14-407-2010. Fu, K.D., He, D.M., 2007. Analysis and prediction of sediment trapping efficiencies of the construction and operation of the planned cascade dams along reservoirs in the mainstream of the Lancang River. Chin. Sci. Bull. 52, 134–140. the upper and middle mainstream of the Lancang River over the next http://dx.doi.org/10.1007/s11434-007-7026-0 (SI). several decades will further alter the river's hydrological regimes and Fu, K.D., Huang, J.Ch., He, D.M., 2008a. Assessment of sediment trapped by the Manwan Dam on Lancang River. J. Sediment. Res. (4), 36–40. http://dx.doi.org/10.3321/j.issn: subsequently affect the downstream environments and ecosystems. 0468-155X.2008.04.007 (In Chinese with English abstract). 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