A 40-year study of a a tropical reservoir aging in real time. The warm surface layer deepens by a meter per decade as the climate heats up.
Chaoborus, CC BY-SA 3.0
A living laboratory
When the Nam Ngum 1 Hydropower dam was completed in 1971, it created the largest body of water in Laos. The reservoir, about 90 kilometers north of Vientiane, submerged 370 square kilometers of forest and land, creating a complex ecosystem with a maximum depth of around 60 meters. This artificial lake functions as a warm monomictic reservoir, meaning its waters mix from top to bottom only once per year, during the cooler, drier season.
Long-term environmental monitoring here provides a rare, multi-decade record of how a tropical reservoir is responding to climate change. The most direct impact is on its thermal structure. Data shows the warm, oxygenated surface layer—the epilimnion—is steadily deepening. This process, driven by rising air temperatures, happens at a rate of approximately one meter every decade. The reservoir's surface waters are getting warmer and pushing deeper into the water column.
A stratified future
The thickening of the warm surface layer intensifies the reservoir's thermal stratification. This makes it much more difficult for wind to mix the water, isolating the deep, cold bottom layer, known as the hypolimnion. For most of the year, this bottom layer becomes completely anoxic, meaning it has zero dissolved oxygen. This anoxia has deep effects on the reservoir's aquatic life and chemistry.
The lack of oxygen effectively shrinks the habitable zone for fish, forcing them into the warmer upper layers. Many of the reservoir's more than 55 indigenous fish species are affected. The low-oxygen water discharged through the dam's turbines also impacts the river downstream, where dissolved oxygen levels often fall below the 5 mg/l threshold considered necessary for healthy aquatic ecosystems. Anoxic conditions at the reservoir floor can cause nutrients like phosphorus to be released from the sediment, a process that fuels eutrophication and can lead to algal blooms.
💡Fun Facts
The reservoir was created by the Nam Ngum 1 dam, the first hydropower project built in Laos, with construction starting in 1968.
The primary purpose of the dam is hydroelectricity generation, and for many years power sold to Thailand was Laos's biggest export earner.
The reservoir flooded a forest, and many tree trunks still stand underwater today, with some having been harvested by underwater logging operations.
The small clupeid fish *Clupeichthys aesarnensis*, known locally as Pa Keo, is a major component of the reservoir's fishery.
