In May, the Tibetan Plateau is still in the depths of winter: There are long stretches of desolate, bleak mountains and rivers flowing from the snowy peaks are still covered in ice.

The river, a tributary of the Zhajia Zangbu River, originates in the glaciers of high peaks on the two sides of the Danggula Mountain pass and finally empties into the Selin Lake -- the second largest lake in Tibet. When it is early spring in Eastern China, in the Tibetan Plateau -- with an elevation of more than 5,000 meters -- the snow on the road has just melted away and the river is still covered in ice.

Having crossed the Danggula pass, a member from the "Walk on Dragon's Back" inspection team of the Qinghai-Tibetan Plateau shouts: "Look, there is a big bump in the river!"

The team members look over to find that a very big bump has disrupted the smooth glacier on the river. They rush towards it for a closer look: The glacier is as wide as the riverbed and more than three meters high. In contrast to the even ice surface in its upper and lower reaches, it seems that the river's "blood vessels" have been blocked, causing a big bump.

"It is a frost mound," explains Li Shuangke, team leader and researcher at the Institute of Geography of the Chinese Academy of Sciences. "At the end of autumn and beginning of winter, the temperature begins to drop and water in rivers ice up. Rivers ice up starting from the surface and a thin ice layer will first appear on the surface. When the temperature continues to drop, water below the ice layer also begins to freeze, and its volume swells up after freezing so the new ice humps up on the old ice. Here, the temperature in winter may reach several tens of degrees Celsius below zero, the river water keeps freezing and swelling and finally a large frost mound like this comes into being."

"Does this mean frost mounds usually appear on rivers in cold areas?" asks a team member.

Li tells the group that a frost mound is a type of frost landform, which usually appears in areas with a high latitude and high elevation. If there is much water in the soil, then frost mounds may also appear; so they do not only appear in rivers. In the Tibetan Plateau, bumps caused by frozen soil are not rare and some new frost mounds emerge each year, causing much difficulty for road and railway construction and maintenance.

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