The hydraulic dam effect
At Pakse, the tranquil mouth of the Xe Don River meets the colossal Mekong, the longest river in Southeast Asia. For most of the year, the Xe Don contributes its flow to the larger river, a typical tributary relationship. But during the monsoon season, which lasts from roughly May to October, this dynamic dramatically inverts. The Mekong, swollen with rainfall from its vast catchment, experiences a massive increase in water volume. This surge raises the Mekong's water level so high that it effectively is a "hydraulic dam" at the confluence. The smaller Xe Don, unable to push against the Mekong's immense pressure, has its own flow stopped and then physically reversed.
This phenomenon, known as a backwater effect, is powerful enough to push water from the Mekong several kilometers back up the Xe Don's channel. It is a seasonal and temporary reversal, entirely driven by the flood pulse of the larger river. Similar, more famous annual reversals occur on a larger scale where the Mekong meets the Tonlé Sap River in Cambodia, causing the Tonlé Sap lake to expand to six times its dry-season size. The event at Pakse is a localized demonstration of how a dominant river can dictate the behavior of its tributaries.
A zone of deposition
The reversal of the Xe Don's flow has a significant geological consequence: sediment deposition. Rivers carry suspended particles of silt, clay, and sand. As long as the water moves with sufficient velocity, this sediment load stays suspended. When the Mekong's floodwaters halt and reverse the Xe Don, the water in the confluence zone slows to a near standstill. This stagnant water can no longer hold its sediment load in suspension.
The result is that the particles drop out and settle on the riverbed at the meeting point. This process deposits an average of 8 millimeters of new sediment each year, building up the local riverbed and altering the channel's morphology. This deposition zone records the annual battle between the two rivers. The sediment itself is important for the river's ecosystem, carrying nutrients that support local agriculture and fisheries downstream. Increased dam construction upstream on the Mekong traps vast quantities of this sediment, with some studies estimating that future developments could prevent up to 96% of the historical sediment load from reaching the lower parts of the river and its delta. This "sediment starvation" threatens the long-term stability and fertility of river systems like the one at Pakse.