Wetlands at 4,500 meters act as sponges storing monsoon rains. Isotope hydrology traces how long water takes to travel from marsh to river mouth, 6,000km away.
Quigley, CC0, via Wikimedia Commons
The river's nursery
On the vast Qinghai-Tibet Plateau, at an average elevation of 4,500 meters, lies the source of Asia's longest river, the Yangtze. This region, known as Sanjiangyuan or "Source of Three Rivers," also gives rise to the Yellow and Mekong rivers. The landscape is a network of high-altitude marshes, alpine meadows, and lakes that form the highest natural wetland in China. This ecosystem is a massive natural sponge, absorbing and storing water from monsoon rains and glacial melt.
The wetlands are underlain by a layer of permafrost, ground that remains frozen for at least two consecutive years. This frozen layer is impermeable, preventing water from seeping deep into the earth and forcing it to collect at the surface, which sustains the marshes. This interplay between permafrost and surface water is fundamental to the region's hydrology. It creates a system that slowly releases water, regulating the flow of the Yangtze River and providing a stable water supply to downstream areas hundreds of kilometers away. Studies show that this supra-permafrost water, held in the active layer above the permafrost, is a primary source of runoff, even more so than direct precipitation in some areas.
Tracking water's long journey
Scientists use the principles of isotope hydrology to trace the movement of water from these source wetlands to the Yangtze's mouth at the East China Sea, a distance of over 6,000 kilometers. Water molecules contain stable isotopes of hydrogen (Deuterium) and oxygen (Oxygen-18). The ratios of these isotopes vary depending on the water's origin and history, creating a natural signature. Rain from monsoons, for example, has a different isotopic signature than meltwater from a glacier.
By sampling water at the source and at various points downstream, researchers can track how long it takes for the water stored in the high-altitude "sponge" to travel the river's length. The analysis reveals how different sources—including recent precipitation, supra-permafrost water, and glacial meltwater—contribute to the river's flow. One study in the source region determined that runoff was composed of roughly 41% precipitation, 41% supra-permafrost water, and 17% glacier and snow meltwater. This data is crucial for understanding how climate change, which is causing permafrost to thaw and glaciers to retreat, will affect water availability for nearly a third of China's population living in the river's basin.
The region's unique ecosystem supports a variety of specially adapted wildlife. It is a vital habitat for endangered species like the Tibetan antelope (Pantholops hodgsonii) and the black-necked crane (Grus nigricollis), the world's only alpine crane. The population of Tibetan antelopes in the area has recovered from under 20,000 to more than 70,000 due to conservation efforts.
💡Fun Facts
The area is part of the Sanjiangyuan National Park, which is roughly the size of England and China's first official national park.
The Tibetan name for the source waters of the Yangtze is Drichu, which means "Water of the Female Yak."
The Tibetan antelope, or Chiru, is unique among antelopes because the females do not have horns.
The black-necked crane breeds at altitudes up to 4,800 meters, making it the only exclusively alpine crane species.
The park is a vast, remote wilderness without set opening hours, but travel is restricted by weather and daylight.
Admission
Access to core areas of the Sanjiangyuan National Park may require special permits and the hiring of a guide.
Accessibility
The terrain is rugged, at extreme high altitude (above 4,000 meters), and undeveloped. Travel is by 4WD vehicle on rough roads. Acclimatization is necessary to avoid severe altitude sickness.
