1. Introduction

Hydropower dams are being developed extensively in developing regions, such as the Congo, Yangtze, Yellow, Amazon, and Mekong Rivers, which have complex tradeoffs among river ecosystems, local people, communities, and economies [1]. For instance, dams provide economic and social benefits, such as flood risk reduction [2,3], irrigation water [4], and electricity [5]. On the other hand, they may alter hydrology [6], destroy wetland destruction [7,8], degrade water quality [9], alter sediment transport [10,11], fragment rivers [12], reduce biological and ecological productivity [13], and contribute to biodiversity loss [14]. Flow alteration is a fundamental change to river systems, because it is a master variable that affects physical, chemical, and biological processes[15]. Typically, dams decrease flows in the wet season and increase them in the dry season [7,16], and magnitude, timing, frequency, duration, and rate of change can be affected [17,18]. Extended wet or dry periods may occur [19,20].

The Mekong River supports tremendous aquatic biodiversity, including ~1,200 fish species [21] and is home to aquatic invertebrates such as rotifers [22], aquatic insects [23,24], annelids, crustacean, and molluscs [25,26]. Fish from the Lower Mekong Basin (LMB) are a primary food source for local people [27,28], and fisheries provided an estimated ~US$11 billion per year to the economy in 2015 [29]. Despite being ecologically, economically, and socially important, the Mekong River is a hydropower dam development hotspot. Since the mid-1990s, 11 mega dams have been constructed on Upper Mekong River mainstem in China [30]. In the LMB, at least 129 dams have been commissioned, including five mainstem dams in Laos. Hydropower dams in the LMB provide capacity for >30,000 megawatts (MW) of power generation, with estimated revenue of US$160 billion from all projects by 2040 [31]. One of the primary areas for hydropower dams in the LMB is the Sekong, Sesan and Srepok Rivers (3S Basin), which are a major tributary to the LMB (Figure 1). As of 2021, at least 51 dams were operating in the 3S Basin, with a combined generating capacity of 4,684 MW. The Sesan River has the most dams of the three basins, with 22 dams, and the Sekong has the fewest dams, with 14 dams [32].

Given on-going hydropower dam development, streamflow in 3S rivers have deviated from natural regimes [33,34]. Piman, Cochrane, Arias, Green and Dat [33], examined the effects of hydrologic change by hydropower dams in the whole 3S, using the water assessment tool (SWAT) and the HEC-ResSim models, and found that the construction of new dams along the main rivers of the 3S basin can alter seasonal flows, leading to increased dry season flow and decreased wet season flow, as part of a strategy to maximize electricity production. Oeurng and Sok [34] evaluated changes in flow and water quality in the Sesan River using the Indicators of Hydrological Alteration (IHA) framework. They discovered significant hydrologic changes occurred during both the low flow and high flow periods following the construction of Yali Falls dam in Vietnam. The IHA model has been widely implemented and is well-known for its effectiveness at quantifying flow changes [17,35,36,37]. For example, Van Binh, Kantoush, Saber, Mai, Maskey, Phong and Sumi [35] utilized IHA to examine the long-term alteration of low regime of the Mekong River. Piman [36] estimated how changes in land use, climate, and hydropower development affect the hydrologic alteration of Srepok River using IHA. Zhou, Huang, Zhao, Ma and Sciences [37] used IHA to analyze the cumulative effects of cascading reservoirs on the flow regime in the Jinsha River in China. Since flow regime is central to sustaining ecosystem function, productivity, and the livelihoods of local people [11,38,39]. Understanding the impact of dam operation on flow changes is crucial for addressing transboundary concerns, including power generation and downstream ecosystem alteration, and can provide insights for better reservoir planning/building. Therefore, it is important to quantify and compare flow alteration before and after dam construction and between the least and most dammed rivers in the 3S.

In this study, we quantify the degree of hydrologic alteration prior to and following dam construction in the Sekong and Sesan Rivers. For each river, we first assess the degree of daily hydrologic alteration between the pre-impact (1965-1968) and post-impact (2018-2021) periods of dam construction using the 5^th and 95^th percentiles of flows to indicate the highest and lowest pulses, respectively. We then compare hydrologic alteration between the two periods for each river and between the two rivers. Low flows, represented by exceedance of 95^th percentile flows, are important for understanding minimum flow requirement for ecological health and managing water resources during dry seasons. Exceedance of the 5^th flow percentile indicates high flows, which are significant for describing flood events.

2. Materials and Methods

2.1. Study Area

The Sekong and Sesan Rivers (2S) are major tributaries of the LMB and are important for irrigation, transportation, hydropower generation, fisheries, and ecosystem services. These rivers flow through portions of Laos, Cambodia, and Vietnam (Figure 1). The total watershed area of the two rivers is about 47,615 km², of which the Sekong Basin encompasses about 28,815 km², while the Sesan Basin is about 18,800 km² [40]. The wet monsoon season from May to October provides more than 80% of the annual rainfall. The dry season lasts from November to April, with cooler temperatures observed from November to January. Average annual rainfall can exceed 2,500 mm and precipitation varies throughout the watersheds. The Sekong, Sesan, and Srepok Rivers collectively contribute a mean annual discharge of ~2,890 m³/s, which is ~25% of streamflow to the Mekong River [41].

2.2. River Flow Data

Daily streamflow was measured by the Mekong River Commission (MRC) at the Siempang gauge station in the Sekong River and the Veurnsai gauge station in the Sesan River. Flow data for each river was divided into two periods: the pre-impact period (PreIm: 1965-1968) and the post-impact period (PostIm: 2018-2021). The pre-impact period was selected because no dams had been built in either river basin and daily data was available. The post-development period followed recent mega hydropower dam construction, including the Se San 4 and Lower Sesan 2 Dams in Sesan River and XeKaman 1 Dam in the Sekong River (Figure 2). The descriptive statistics of streamflow for the pre- and post-impact periods in both rivers is provided in Table A1.

2.3. Data Analysis

2.3.1. Indicators of Hydrologic Alteration and Extreme Flow Events

We used the IHA framework to quantify streamflow changes from hydropower dam development at study stream gauges. The IHA framework was created to assess streamflow alteration by detecting differences in the range of natural variability, using 33 parameters of hydrologic alteration [42,43]. The pre-impact period is our reference condition for natural flows. In this study, we used only 10 of 33 parameters (Table 1) for the subsequent analysis identifying the extreme flow events.

We used flow duration curves to assess changes in the low and high magnitude flows. The 5^th (Q₅) and 95^th (Q₉₅) exceedance probabilities of flow in each period were calculated, where Q₅ indicates high magnitude flows and Q₉₅ represents low magnitude flows. Q₅ is a common metric to assess how hydrologic alteration affects high flow conditions, such as evaluating the capacity of reservoirs to store large inflows [44]. To identify extreme flow events, we also used 1-, 3-, 7-, 30-, and 90-day minimum and maximum flows estimated by the IHA framework. These indicators are widely used [45].

2.3.2. Hydrologic Alteration Between Periods and Rivers

To examine hydrologic changes between two periods, relative hydrologic change was computed as percentage. The results from this analysis were then were divided into three categories based on their relationship to the median: those less than or equal to the 33^rd percentile, those between the 34^th and 67^th percentiles, and those exceeding the 67^th percentile. These categories represent low, medium, and high degree of hydrologic alteration, respectively [43]. The relative hydrologic change was as following:

R e l a t i v e H y d r o l o g i c C h a n g e = \frac{(P o s t - I m p a c t F l o w) - (P r e - I m p a c t F l o w)}{P r e - I m p a c t F l o w} \times 100

(1)

When the values of the relative hydrologic change is positive, it means that streamflow increased from the pre-impact to the post-impact period. Conversely, a negative value indicates a decrease in the streamflow of the studied rivers [46].

Moreover, flow data was also compared between the pre- and post-impact periods, between rivers, and between the wet and dry seasons (Figure A1) using the Kruskal-Wallis test. In total, six comparative analyses were completed (Table 2).

3. Results

3.1. Changes in Flow Duration Curves and Flow Maxima and Minima

There was a larger reduction in streamflow between the pre- and post-impact periods for Q₅ (the highest 5% of flows) than for Q₉₅ (the lowest 5% of flows), and for the Sekong River than the Sesan River (Figure 3). The inset table notes the magnitude of low and high flows during the pre- and post-impact periods for the Sekong and Sesan Rivers.

Overall, the Sekong River—with the fewest dams—had larger flow increases before and after dam development than the Sesan River, that had fewer dams (Figure 4). The 1-, 3-, 7-, 30-, and 90-day minimum flows increased in the Sekong River by more than 250% following dam construction (Figure 4). Minimum flows in the post-impact period were 359 – 425 m³/s higher than the pre-impact period. The 1-, 3-, 7-, and 30-day maximum flows were 50% higher than the pre-dam impact period, while there was a negligible change for 90-day maximum flow (Figure 4). For the Sesan River, minimum flows increased by 120 – 160% in the post-dam impact period. The 1-day maximum flow had a 56% increase, with smaller increases for the 3-, 7-, and 30-day maximum flows. The 90-day maximum flow in the Sesan River was the only minima/maxima metric that decreased following dam construction (Figure 4).

3.2. Hydrologic Alteration between Periods and River Basins

Streamflow in the Sekong River increased in October through May following dam construction, with an average dry season (Nov-Apr) increase of 200% (Table 3). On average, April had the largest increase in streamflow between the pre-and post-dam construction periods. Streamflow decreased from pre-dam conditions from June to September in the wet season, during which July had the greatest reduction (-37%). For the Sesan River, streamflow increased from pre-dam conditions from September through May, with a mean dry season increase of 100%. Flow decreased from pre-dam conditions from June to August, during the wet season. Streamflow decreased most in July in the Sesan River.

Streamflow increased significantly between the pre- and post-impact periods for both the Sekong and Sesan Rivers in the dry season (Figure 5, Table 3). There was no significant change in streamflow between the pre- and post-impact periods for either river in the wet season. Streamflow in the post-impact period was significantly higher in Sekong River than the Sesan River in the dry season.

4. Discussion

Overall, we found that the post-impact flows are generally higher than pre-impact flows for both rivers. Binh et al. [35], based on a long-term data analysis of the Mekong River, also revealed that flow regime alteration was more pronounced in the high-dam development period compared to the no-dam development period. This could be due to substantial change of the flood peak, flood frequency, flood duration, and high-flow discharges induced by dams [35]. Tian et al. [47] also indicate a similar finding from the Three Gorges Dam located in the Yangtze River of China.

Sekong River had a higher volume of flows for both the Q₅ and Q₉₅, compared to Sesan River. This finding is relatively similar to the study of Oeurng et al. [41] who found that the annual flow of 1,167 m³/s of the Sekong River was far higher than that of 743 m³/s for the Sesan River. Higher precipitation in Sekong during dry and wet seasons also leads higher Q₅ and Q ₉₅ [41], as in the case of other studies [48,49]. For the Sesan River, it is the most dammed river of the 3S Basin [32], with six mainstem dams that have capacity >100 Megawatts, including Plei Krong, Yali Falls, Sesan 3, Sesan 3A, Sesan 4, Sesan 4A (63 Megawatts ) and Lower Sesan 2 Dams. Therefore, water can be trapped in the upstream dams located above the stream gauge station (except for Lower Sesan 2) (Figure A2), and therefore contributed to higher volume of flow in overall.

For both rivers, streamflow significantly increased between pre-dam and post-dam periods in dry season. The evidence of increased dry season flow and low-flow also occur following hydropower dam development in the other parts of the Mekong River Basin [35,50] and the Yangtze River of China [47]. This is because dam discharge retained water from the wet season to generate energy production. For instance, between March and May 2016, an amount of 12.65 billion cubic meters of water was released from Jinghong hydropower in Yunnan province of China, contributing to an increased 602-1,010m³/s along the downstream Mekong mainstem river. For the wet season, however, we found no significant flow change whilst previous studies have indicated decreasing wet seasonal flow due to dam operations [39,51,52]. This discrepancy can be subjected to the studies period, dam scenarios, and reservoir operation rules.

5. Conclusion

Overall, we found likely impacts of hydropower dam development on streamflow in the two river basins, most notably as significantly increased dry season flows and low flow minima metrics. We also found significant lower streamflow in Sesan River than in Sekong River. Size of watershed, annual precipitation and extreme climate condition explain differences in streamflow between the two basins.

As the 3S basin is vital for biodiversity, ecosystem services and local communities, limiting future dam development to maintain current riverine migration corridors and habitat connectivity should be a priority.

Author Contributions

Conceptualization: R.K. and R.S.; methodology and formal analysis: R.K., K.C., R.S. and T.S.; investigation: S.T., C.O. and R.S.; data acquisition R.S.; data curation: R.S. and R.K. writing—original draft preparation: R.K. and R.S.; writing—review and editing: R.K., T.S., K.C., S.R.P, C.O. and R.S.; visualization: R.K., K.C. and R.S.; supervision: T.S., C.O. and R.S.; funding acquisition: R.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the United States Agency for International Development’s ‘Wonders of the Mekong’ Cooperative Agreement No: AID-OAA-A-16-00057.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgment

Authors would like to thank the Mekong River Commission for providing the database for our analysis. We are also grateful for the technical support by HydroMet and Disaster Management Lab, Institute of Technology of Cambodia, Phnom Penh, Cambodia.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A

Figure A1. Monthly average discharge at gauge stations in the Sekong and Sesan Rivers.

Figure A2. Cumulative storage of upstream dams above the studied gauge station in Sesan River.

Table A1. Descriptive statistics of streamflow for the pre- and post-impact periods in the Sekong and Sesan Rivers (m³/s).

	Dry Season				Wet Season
	Sekong		Sesan		Sekong		Sesan
	Pre-	Post-	Pre-	Post-	Pre-	Post-	Pre-	Post-
Mean	226.5	708.3	248.1	415.0	1903.0	2085.3	1023.2	1093.6
Median	202.0	654.0	177.0	357.4	1540.0	1374.0	1020.0	840.7
SD	132.0	304.3	172.1	194.0	1387.3	1965.2	669.3	898.7
Min	68.0	281.0	75.0	193.3	135.0	255.0	95.0	225.2
Max	1020.0	3956.0	855.0	1377.3	6130.0	10301.0	3420.0	6101.5
Range	952.0	3675.0	780.0	1184.0	5995.0	10046.0	3325.0	5876.4

References

Moran, E. F.; Lopez, M. C.; Moore, N.; Müller, N.; Hyndman, D. W. Sustainable hydropower in the 21st century. Proceedings of the National Academy of Sciences 2018, 115, 11891–11898. [Google Scholar] [CrossRef] [PubMed]
Agostinho, A. A.; Gomes, L. C.; Veríssimo, S.; K. Okada, E., Flood regime, dam regulation and fish in the Upper Paraná River: effects on assemblage attributes, reproduction and recruitment. Reviews in Fish biology and Fisheries 2004, 14, 11–19. [Google Scholar] [CrossRef]
Nakamura, F.; Komiyama, E. A challenge to dam improvement for the protection of both salmon and human livelihood in Shiretoko, Japan’s third Natural Heritage Site. Landscape Ecological Engineering 2010, 6, 143–152. [Google Scholar] [CrossRef]
Kingsford, R. T. Ecological impacts of dams, water diversions and river management on floodplain wetlands in Australia. Austral Ecology 2000, 25, 109–127. [Google Scholar] [CrossRef]
Baxter, R. ; systematics, Environmental effects of dams and impoundments. Annual review of ecology 1977, 8, 255–283. [Google Scholar] [CrossRef]
Dang, T. D.; Cochrane, T. A.; Arias, M. E.; Van, P. D. T.; de Vries, T. T. Hydrological alterations from water infrastructure development in the Mekong floodplains. Hydrological processes 2016, 30, 3824–3838. [Google Scholar] [CrossRef]
Hecht, J. S.; Lacombe, G.; Arias, M. E.; Dang, T. D.; Piman, T. Hydropower dams of the Mekong River basin: A review of their hydrological impacts. Journal of Hydrology 2019, 568, 285–300. [Google Scholar] [CrossRef]
Kingsford, R.; Thomas, R. Destruction of wetlands and waterbird populations by dams and irrigation on the Murrumbidgee River in arid Australia. Environmental management 2004, 34, 383–396. [Google Scholar] [CrossRef] [PubMed]
Wei, G.; Yang, Z.; Cui, B.; Li, B.; Chen, H.; Bai, J.; Dong, S. Impact of dam construction on water quality and water self-purification capacity of the Lancang River, China. Water resources management 2009, 23, 1763–1780. [Google Scholar] [CrossRef]
Wu, X.; Xiang, X.; Chen, X.; Zhang, X.; Hua, W. Effects of cascade reservoir dams on the streamflow and sediment transport in the Wujiang River basin of the Yangtze River, China. Journal Inland Waters 2018, 8, 216–228. [Google Scholar] [CrossRef]
Klaver, G.; van Os, B.; Negrel, P.; Petelet-Giraud, E. Influence of hydropower dams on the composition of the suspended and riverbank sediments in the Danube. Journal Environmental Pollution 2007, 148, 718–728. [Google Scholar] [CrossRef] [PubMed]
Soukhaphon, A.; Baird, I. G.; Hogan, Z. S. The impacts of hydropower dams in the Mekong River Basin: A review. Water 2021, 13, 265. [Google Scholar] [CrossRef]
Franklin, K.; Dietrich, W.; Trush, W. Downstream ecological effects of dams, a geomorphic perspective. Journal Bioscience 1995, 45, 183–192. [Google Scholar]
Wu, H.; Chen, J.; Xu, J.; Zeng, G.; Sang, L.; Liu, Q.; Yin, Z.; Dai, J.; Yin, D.; Liang, J. Effects of dam construction on biodiversity: A review. Journal of cleaner production 2019, 221, 480–489. [Google Scholar] [CrossRef]
Poff, N. L.; Zimmerman, J. K. Ecological responses to altered flow regimes: a literature review to inform the science and management of environmental flows. Journal Freshwater biology 2010, 55, 194–205. [Google Scholar] [CrossRef]
Habit, E.; García, A.; Díaz, G.; Arriagada, P.; Link, O.; Parra, O.; Thoms, M. ; Applications, River science and management issues in Chile: Hydropower development and native fish communities. River Research 2019, 35, 489–499. [Google Scholar] [CrossRef]
Richter, B. D.; Baumgartner, J. V.; Powell, J.; Braun, D. P. A method for assessing hydrologic alteration within ecosystems. Conservation biology 1996, 10, 1163–1174. [Google Scholar] [CrossRef]
McMillan, H. K. A review of hydrologic signatures and their applications. Journal Wiley Interdisciplinary Reviews: Water 2021, 8, e1499. [Google Scholar] [CrossRef]
Magilligan, F. J.; Nislow, K. H. Changes in hydrologic regime by dams. Journal Geomorphology 2005, 71, 61–78. [Google Scholar] [CrossRef]
Daniels, M. E.; Danner, E. M. The drivers of river temperatures below a large dam. Journal Water Resources Research 2020, 56, e2019WR026751. [Google Scholar] [CrossRef]
Rainboth, W. J.; Vidthayanon, C.; Yen, M. D., Fishes of the greater Mekong ecosystem with species list and photographic atlas. 2012.
Sor, R.; Meas, S.; Wong, K. K.; Min, M.; Segers, H. Diversity of Monogononta rotifer species among standing waterbodies in northern Cambodia. Journal of Limnology.
Doeurk, B.; Chhorn, S.; Sin, S.; Phauk, S.; Sor, R. Diversity, distribution and habitat associations of aquatic beetles. Cambodian Journal of Natural History 2022, 90. [Google Scholar]
Chhorn, S.; Chan, B.; Sin, S.; Doeurk, B.; Chhy, T.; Phauk, S.; Sor, R. Diversity, abundance and habitat characteristics. Cambodian Journal of Natural History 2020, 61. [Google Scholar]
Sor, R.; Boets, P.; Chea, R.; Goethals, P. L.; Lek, S. Spatial organization of macroinvertebrate assemblages in the Lower Mekong Basin. Limnologica 2017, 64, 20–30. [Google Scholar] [CrossRef]
Sor, R.; Ngor, P. B.; Boets, P.; Goethals, P. L.; Lek, S.; Hogan, Z. S.; Park, Y.-S. Patterns of mekong mollusc biodiversity: Identification of emerging threats and importance to management and livelihoods in a region of globally significant biodiversity and endemism. Water 2020, 12, 2619. [Google Scholar] [CrossRef]
Ngor, P. B.; Sor, R.; Prak, L. H.; So, N.; Hogan, Z. S.; Lek, S. In Mollusc fisheries and length–weight relationship in Tonle Sap flood pulse system, Cambodia, Annales de Limnologie-International Journal of Limnology, 2018; EDP Sciences: 2018; p 34.
Ziv, G.; Baran, E.; Nam, S.; Rodríguez-Iturbe, I.; Levin, S. A. Trading-off fish biodiversity, food security, and hydropower in the Mekong River Basin. Proceedings of the National Academy of Sciences 2012, 109, 5609–5614. [Google Scholar] [CrossRef]
Hortle, K.; Bamrungrach, P. Fisheries habitat and yield in the Lower Mekong Basin. MRC technical paper 2015, 47, 1–69. [Google Scholar]
Fan, H.; He, D.; Wang, H. Environmental consequences of damming the mainstream Lancang-Mekong River: A review. Earth-Science Reviews 2015, 146, 77–91. [Google Scholar] [CrossRef]
Franciscato, B. R. F.; Udaeta, M. E. M.; Gimenez, A. L. V.; de Almeida Prado Jr, F. A. Strategic decision making based on energy rates and costs from Itaipu's Multinational Energy Agreement Perspective. Energy Strategy Reviews 2022, 44, 100967. [Google Scholar] [CrossRef]
Sor, R.; Ngor, P. B.; Lek, S.; Chann, K.; Khoeun, R.; Chandra, S.; Hogan, Z. S.; Null, S. E. Fish biodiversity declines with dam development in the Lower Mekong Basin. Scientific Reports 2023, 13, 8571. [Google Scholar] [CrossRef]
Piman, T.; Cochrane, T.; Arias, M.; Green, A.; Dat, N., Assessment of flow changes from hydropower development and operations in Sekong, Sesan and Srepok Rivers of the Mekong Basin. 2013.
Oeurng, C.; Sok, T. Assessing changes in flow and water quality emerging from hydropower development and operation in the Sesan River Basin of the Lower Mekong Region. Journal of Hydrology 2020, 6, 1–12. [Google Scholar]
Van Binh, D.; Kantoush, S. A.; Saber, M.; Mai, N. P.; Maskey, S.; Phong, D. T.; Sumi, T. Long-term alterations of flow regimes of the Mekong River and adaptation strategies for the Vietnamese Mekong Delta. Journal of Hydrology: Regional Studies 2020, 32, 100742. [Google Scholar]
Piman, T., Multiple drivers of hydrological alteration in the transboundary Srepok River Basin of the Lower Mekong Region. 2020.
Zhou, X.; Huang, X.; Zhao, H.; Ma, K. J. H.; Sciences, E. S., Development of a revised method for indicators of hydrologic alteration for analyzing the cumulative impacts of cascading reservoirs on flow regime. 2020, 24, 4091–4107.
Winton, R. S.; Calamita, E.; Wehrli, B. Reviews and syntheses: Dams, water quality and tropical reservoir stratification. Journal Biogeosciences 2019, 16, 1657–1671. [Google Scholar] [CrossRef]
Piman, T.; Lennaerts, T.; Southalack, P. Assessment of hydrological changes in the lower Mekong Basin from Basin-Wide development scenarios. Journal Hydrological Processes 2013, 27, 2115–2125. [Google Scholar] [CrossRef]
Constable, D., Atlas of the 3S Basins. 2015.
Oeurng, C.; Cochrane, T. A.; Arias, M. E.; Shrestha, B.; Piman, T. Assessment of changes in riverine nitrate in the Sesan, Srepok and Sekong tributaries of the Lower Mekong River Basin. Journal of Hydrology: Regional Studies 2016, 8, 95–111. [Google Scholar] [CrossRef]
Richter, B.; Baumgartner, J.; Wigington, R.; Braun, D. How much water does a river need? Freshwater biology 1997, 37, 231–249. [Google Scholar] [CrossRef]
Richter, B. D.; Baumgartner, J. V.; Braun, D. P.; Powell, J. A spatial assessment of hydrologic alteration within a river network. Regulated Rivers: Research Management: An International Journal Devoted to River Research Management 1998, 14, 329–340. [Google Scholar] [CrossRef]
Yang, T.; Zhang, Q.; Chen, Y. D.; Tao, X.; Xu, C. y.; Chen, X. A spatial assessment of hydrologic alteration caused by dam construction in the middle and lower Yellow River, China. Hydrological Processes: An International Journal 2008, 22, 3829–3843. [Google Scholar] [CrossRef]
Wang, Y.; Wang, D.; Lewis, Q. W.; Wu, J.; Huang, F. A framework to assess the cumulative impacts of dams on hydrological regime: A case study of the Yangtze River. Hydrological Processes 2017, 31, 3045–3055. [Google Scholar] [CrossRef]
Conservancy, N., Indicators of hydrologic alteration. Version 7. User’s manual. The Nature Conservancy 2007.
Tian, J.; Chang, J.; Zhang, Z.; Wang, Y.; Wu, Y.; Jiang, T. Influence of Three Gorges Dam on downstream low flow. Water 2019, 11, 65. [Google Scholar] [CrossRef]
Hope, A.; Bart, R. Synthetic monthly flow duration curves for the Cape Floristic Region, South Africa. Water SA 2012, 38, 191–200. [Google Scholar] [CrossRef]
Abimbola, O. P.; Wenninger, J.; Venneker, R.; Mittelstet, A. R. The assessment of water resources in ungauged catchments in Rwanda. Journal of Hydrology: Regional Studies 2017, 13, 274–289. [Google Scholar] [CrossRef]
Mekong River Commission (MRC), Hydrological Impacts of the Lancang Hydropower Cascade on Downstream Extreme Events. Mekong River Commission Secretariat: Vientiane, Laos 2019.
Liu, K.-T.; Tseng, K.-H.; Shum, C. K.; Liu, C.-Y.; Kuo, C.-Y.; Liu, G.; Jia, Y.; Shang, K., Assessment of the Impact of Reservoirs in the Upper Mekong River Using Satellite Radar Altimetry and Remote Sensing Imageries. 2016, 8, (5), 367.
Do, P.; Tian, F.; Zhu, T.; Zohidov, B.; Ni, G.; Lu, H.; Liu, H. Exploring synergies in the water-food-energy nexus by using an integrated hydro-economic optimization model for the Lancang-Mekong River basin. Science of the Total Environment 2020, 728, 137996. [Google Scholar] [CrossRef] [PubMed]

Figure 1. The Sekong and Sesan Rivers, with dams and monitoring stations.

Figure 2. Cumulative reservoir storage for both river basins (gray shading), the Sekong River (blue line) and Sesan River (red dashed line), with a timeline of dam construction.

Figure 3. Flow duration curves for the pre- and post-impact periods at the Siempang station in the Sekong River and the Veurnsai station in the Sesan River.

Figure 4. Percent change of post-dam development flow minima and maxima compared to pre-dame development hydrology at the Siempang station in the Sekong River and the Veurnsai station in the Sesan River.

Figure 5. Box and whisker plots showing the differences in streamflow between the pre- and post-impact period for each river, and between rivers in dry and wet seasons.

Table 1. Summary of hydrologic parameters extracted from the analyzed IHA framework and their characteristics. From Richter et al. [42].

IHA Statistics Group	Regime Characteristics	Streamflow parameter used in this study
Magnitude and duration of annual extreme water conditions	Magnitude, duration	Annual minima 1-day means Annual maxima 1-day means Annual minima 3-day means
		Annual maxima 3-day means
		Annual minima 7-day means
		Annual maxima 7-day means
		Annual minima 30-day means
		Annual maxima 30-day means
		Annual minima 90-day means
		Annual maxima 90-day means

Table 2. Summary of comparative analyses made for this study.

Pre- vs Post-Impact Comparison	Basin Comparison
(1) Dry-season Sekong River Pre- vs Post-Hydropower Impact	(5) Sekong vs Sesan
(2) Dry-season Sesan River Pre- vs Post-Hydropower Impact	(5) Sekong vs Sesan
(3) Wet-season Sekong River Pre- vs Post- Hydropower Impact	(6) Sekong vs Sesan
(4) Wet-season Sesan River Pre- vs Post- Hydropower Impact	(6) Sekong vs Sesan

Table 3. Average monthly flow and relative change for the pre- and post-impact periods at the Siempang station in the Sekong River and the Veurnsai station in the Sesan River. Orange and green highlighted correspond to the dry and wet seasons, respectively.

Month	Sekong				Sesan
Month	Pre- (m³/s)	Post- (m³/s)	Relative Change (%)	Degree of Change	Pre- (m³/s)	Post- (m³/s)	Relative Change (%)	Degree of Change
Nov	586	747	27	Low	408	723	77	High
Dec	396	632	60	Medium	330	496	50	Medium
Jan	278	667	140	High	234	364	55	Medium
Feb	209	627	200	High	157	284	81	High
Mar	136	567	315	High	119	305	156	High
Apr	100	557	457	High	116	326	181	High
Dry Season Mean ±SD	284 ±182	633 ±70	200 ±163	High	227 ±120	416 ±168	100 ±55	High
May	234	565	142	High	133	387	191	High
Jun	692	675	-3	Low	640	501	-21	Low
Jul	1,695	1,075	-37	Medium	1,275	855	-33	Low
Aug	2,425	2,406	-1	Low	1,400	1,151	-18	Low
Sep	2,950	2,891	-2	Low	1,482	1,766	19	Low
Oct	1,032	1,178	14	Low	633	986	56	Medium
Wet Season Mean ±SD	1,505±1,046	1,465 ±958	18.8 ±63	Low	927 ±539	941 ±497	32 ±84	Low
Annual Mean ±SD	894±958	1,049 ±780	109 ±151	High	557 ±522	679 ±447	66 ±77	High

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

© 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).