As hydropower dams quell the Mekong’s life force, what are the costs?
Abstract from this article by mongabay.com:
The river yields one-fifth of the world’s total freshwater fish catch every year. More than 1,000 types of freshwater fish migrate seasonally up and down the river, fueling wild fisheries that provide food security and livelihoods for tens of millions of people.
To date, more than 160 hydropower dams operate on the river and its tributaries, including 13 on the river’s mainstream, with hundreds more either planned or under construction.
The region experienced a string of abnormally hot and dry years between 2019 and 2021, for instance, during which time the river ebbed to its lowest level on record.
Monitoring initiatives focused on the region are now documenting evidence of the ways hydropower dams impact the river. Findings show that dams are unequivocally affecting the driving force behind the river system’s impressive ecological richness: its wet season/dry season ebb and flow.
The Mekong’s seasonal ebb and flow, often referred to as its “flood pulse,” is deeply connected to the region’s monsoonal climate. During the rainy season, typically from June to November, the whole river system transitions into a flood phase. Swollen by rainwater, the river inundates surrounding land, depositing nourishing sediment on floodplains and transforming forests into fish nurseries.
By the time the flood pulse reaches Phnom Penh in Cambodia, the water flow is so great that it reverses the flow of Tonle Sap River, channeling water north into Tonle Sap Lake, which can expand up to five times its dry-season size to cover an area half the size of Belgium. The fish nursery and spawning grounds in the great lake are so productive that fish caught in the lake provide the people of Cambodia with up to 70% of their dietary protein.
Conversely, the floodwaters recede with the arrival of the dry season in November, exposing newly fertilized floodplains and riverbanks on which migratory birds lay eggs and communities up and down the river grow crops, tend livestock and host events.
Typically operated to optimize electricity generation, the dams require a steady year-round water flow. As a result, dam operators restrict water to build up reservoirs during the wet season, when the river should be in full flow, and release water during the dry season, when the river’s natural flow should be waning.
Since December 2020, the Mekong Dam Monitor, a collaborative initiative between the U.S.-based Stimson Center and satellite research consultancy Eyes on Earth, has gathered evidence on how hydropower dams are altering the natural flow of the river.
The first complete year of Mekong Dam Monitor data indicated that hydropower dams have “inexorably” altered the river’s natural flow, with large dams compounding water shortages in downstream parts of the basin during some of the worst drought years in recorded history between 2019 and 2021, according to an analysis released in 2022.
The 2022 analysis showed that during the delayed onset of monsoon rains in 2020, river levels in the lower basin hit an all-time low and Tonle Sap Lake remained at dry-season levels during the height of the wet season when it should have been in full flood.
The Mekong Dam Monitor report also indicated that unseasonal water releases by upstream dams are causing abnormally high dry-season river levels, with Stung Treng one of the most severely impacted locations. Fish, birds and plants thrive in the flooded forests here, but riverside communities in the area report that since 2015, ancient trees have been dying out, likely due to year-round submersion.
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