A river's ancient detours
The Jinshajiang, or "Gold Sand River," is the name for the upper section of the Yangtze River. In the mountainous terrain of Yunnan province, this powerful river has not always followed its present-day channel. Geological evidence shows the river has dramatically changed its path in a process called avulsion, where a river abruptly abandons its channel to flow along a new one. This part of China sits on the southeastern margin of the Tibetan Plateau, a region shaped by the ongoing collision of the Indian and Eurasian tectonic plates. This intense geological activity causes rapid uplift and faulting, led to large-scale landscape changes, including the rerouting of entire river systems.
Before establishing its modern course, the upper Yangtze did not flow eastward. Instead, a paleo-Jinshajiang River likely flowed south, joining other rivers and heading toward the Indochina block. Geologists have identified abandoned river channels and thick deposits of lake sediments that show these ancient avulsion events. These features show a landscape in flux, where rivers were captured and redirected by evolving topography.
The sedimentary record of a valley jump
Near the famous First Bend of the Yangtze, geological studies have uncovered the history of these dramatic shifts. One major event involved a landslide that blocked the river around 79,300 years ago. This natural dam created a large lake upstream in an area called the Yinduba paleolandslide-dammed lake. This lake existed for at least 20,000 years before the dam was breached and the river established a new path. Another landslide-damming event in the region occurred around 60,000 years ago, also forming a short-lived lake.
Scientists determine the age of these events by studying the layers of fine-grained sediment left behind by the ancient lakes. Using a technique called Optically Stimulated Luminescence (OSL) dating, they can measure the last time mineral grains, like quartz, were exposed to sunlight. When sediment is buried in a lakebed, it is shielded from light. The OSL method measures the amount of background radiation the quartz grains have absorbed since burial, effectively showing how long they have been hidden in darkness. These dates provide a precise timeline for when the river was forced from its old channel and began carving a new one, eventually creating dramatic features like the Tiger Leaping Gorge.