A river's sudden shift
Along the border of Thailand and Laos, the landscape of Nakhon Phanom Province holds a subtle but immense scar. This scar is a paleochannel, the remnant of a path the Mekong River once took. Around 1,200 years ago, the twelfth-longest river in the world abruptly abandoned this section of its course and carved a new one. This process, known as avulsion, left behind a series of oxbow lakes and a dry, curving depression, a remnant of the river's former might.
The location of this dramatic change is the Khorat Plateau, a vast, gently undulating tableland that covers much of northeastern Thailand. This plateau, composed mainly of Mesozoic sandstones and shales, slopes gently towards the Mekong. Over millennia, the river has deposited deep layers of alluvial sediment across this plain. An avulsion occurs when a river's channel becomes choked with its own sediment. This build-up raises the riverbed until, during a major flood, the water breaches the natural levees and finds a shorter, steeper path. The old channel is left behind, slowly filling with silt and vegetation. Satellite imagery clearly shows the abandoned meander near Nakhon Phanom, a permanent record of a single, powerful event in the river's long history.
Climate as the culprit
The timing of this major avulsion corresponds with the Medieval Warm Period (also called the Medieval Climate Anomaly), a global climate event that lasted from roughly 900 to 1300 CE. This period's effects were not uniform across the globe; while Europe experienced unusually clement weather, other regions saw deep changes in precipitation. For Southeast Asia, evidence suggests the era brought intense droughts punctuated by more powerful monsoon floods.
Scientists theorize that these climatic extremes led to the Mekong's course correction. A series of prolonged droughts would have diminished the river's flow, allowing vast quantities of sediment to accumulate and clog the channel. When the powerful monsoon rains finally returned, the resulting floodwaters would have carried enormous energy. Unable to follow its blocked, elevated path, the river burst its banks and carved a more efficient route across the floodplain.
To date these ancient river deposits, geologists use a technique called Optically Stimulated Luminescence (OSL). OSL dating measures the last time sand grains were exposed to sunlight. By taking core samples from the sediment that filled the abandoned channel, scientists can determine when it was suddenly cut off from the main river flow, pinpointing the date of the avulsion. This method is an effective tool for connecting landscape changes to ancient climate events.