109 © Springer Nature Singapore Pte Ltd. 2019 X. Yu et al., Balancing River Health and Hydropower Requirements in the Lancang River Basin, https://doi.org/10.1007/978-981-13-1565-7_4 Chapter 4 Transboundary Environmental Effects of Hydropower: Hydrology The Lancang River Basin in China is undergoing extensive hydropower develop- ment, and several large dams on the upper Lancang-Mekong River have become operational in the last 20 years. The Lancang-Mekong River fows through Myanmar, Laos, Thailand, Cambodia, and Vietnam after leaving China; thus impacts of the development of the Chinese Lancang cascade of hydropower projects may extend well beyond national boundaries. Transboundary effects may be both positive and negative. For example, although natural ecosystems have the potential to be nega- tively impacted, water management may alleviate some of the negative effects of drought or food. A number of studies have investigated the transboundary environmental effects of the cascade of dams on the mid-lower mainstem of the Lancang River (Lu and Siew 2006; Campbell 2007; Adamson et al. 2009; Lu et al. 2014; Fan et al. 2015). However, only short-term, aperiodic, and sporadic in situ observational data are available (Fan et al. 2015); thus, considerable uncertainties and knowledge gaps remain when evaluating the environmental impacts of the cascade of dams on the Lancang River. In particular, there are few studies of the transboundary effects of China’s dams on the Lancang River following the construction of two projects with large reservoirs (i.e., Xiaowan and Nuozhadu). The study of transboundary environmental effects of hydropower projects on the Lancang River was a key component of this book. Four key transboundary effects were considered: hydrology (discharge and water level), water temperature, sedi- ment transport and geomorphology, and fsh community. The approach used for assessing the four key transboundary effects differed. Some effects were investi- gated through data collection and analysis, some relied on literature review, and others used a combination of approaches. To assess effects on hydrology, hydrological data from monitoring stations on the Lancang and Mekong rivers from 1960 to 2015 were divided into before and after periods (before and after dam construction), and a comparison of results for hydrological stations located at varying distances downstream was used to evaluate transboundary effects on water level and discharge. Results were also compared to