River Discharge and Water Level Changes in the Mekong River: Droughts in
an Era of Mega-Dams
Abstract
While 1992 marked the first major dam – Manwan – on the main stem of
the Mekong River, the post-2010 era has seen the construction and
operationalisation of mega dams such as Xiaowan (started operations in
2010) and Nuozhadu (started operations in 2014) that were much larger
than any dams built before. The scale of these projects implies that
their operations will likely have significant ecological and
hydrological impacts from the Upper Mekong Basin to the Vietnamese Delta
and beyond. Historical water level and water discharge data from 1960 to
2020 were analysed to examine the changes to streamflow conditions
across three time periods: 1960-1991 (pre-dam), 1992-2009 (growth) and
2010-2020 (mega-dam). At Chiang Saen, the nearest station to the China
border, monthly water discharge in the mega-dam period has increased by
up to 98% during the dry season and decreased up as much as -35%
during the wet season when compared to pre-dam records. Similarly,
monthly water levels also rose by up to +1.16m during the dry season and
dropped by up to -1.55m during the wet season. This pattern of
hydrological alterations is observed further downstream to at least
Stung Treng (Cambodia) in our study, showing that Mekong streamflow
characteristics have shifted substantially in the post-2010 era. In
light of such changes, the 2019-2020 drought – the most severe one in
the recent history in the Lower Mekong Basin – was a consequent of
constructed dams reducing the amount of water during the wet season.
This reduction of water was exacerbated by the decreased monsoon
precipitation in 2019. Concurrently, the untimely operationalisation of
the newly opened Xayaburi dam in Laos coincided with the peak of the
2019-2020 drought and could have aggravated the dry conditions
downstream. Thus, the mega-dam era (post-2010) may signal the start of a
new normal of wet-season droughts.