Transboundary water governance is multiscale and multilevel, involving different actors with diverse interests, power and politics at different levels and scales. The specific level and scale are key governance challenges in transboundary water governance. Although there is considerable literature on the scale and level, there is a little elaboration on how they are operationalized in transboundary water governance. Accordingly, the present study focuses on how scale and level complicate transboundary water governance. It is based on literature and a case study on Tonle Sap Lake (TSL) to illustrate the implications of transboundary water governance at different levels and scales on the Mekong River Basin (MRB) that have also affected water governance of TSL. The Mekong River is the 10th largest river and 12th longest river in the world. Its basin drains a total land area of 795,000 km 2 from the eastern watershed of the Tibetan Plateau to the Mekong Delta. The Mekong River flows approximately 4909-km through three provinces of China, continuing into Myanmar, Lao PDR, Thailand, Cambodia and Viet Nam before emptying into the South China Sea. It begins in the mountains on the northeastern rim of the Tibetan Plateau in wetlands situated about 5000-m above sea level. It flows through Chinese territory in Qinghai, Tibet and Yunnan for nearly 2200-km. By the time the river leaves China, its altitude has fallen to about 400-m above sea level (Hori, 2000). It then winds its way for just over 2700-km through Myanmar, Laos, Thailand, Cambodia and Vietnam before emptying into the South China Sea (Jacobs, 2002). The Mekong River releases 475 km 3 of water each year to the China Sea (Kummu et al., 2008).