Characteristics of thermokarst lakes and their influence on permafrost in Qinghai–Tibet Plateau

About 250 thermokarst lakes, with an average size of 5580 m2 and a total size of 139 × 104 m2, are spread between the Kunlun Mountain pass and the Fenghuo Mountain pass along the Qinghai–Tibet railway, where ice-rich and warm permafrost exists. Approximately 56% of the lakes are elliptical and 23% are elongated. The water depth varies between 0.4 and 3 m. Most of the lakes in the Chumarhe High Plain and other mountain regions are shallower than the thickness of winter ice (60 to 80 cm), and are frozen to the bottom during the cold seasons. However, in the Hoh Xil Hill region and Beiluhe basin, the water depth is greater than the thickness of winter ice and the relatively warm lake bottoms can cause considerable disturbance to the surrounding continuous permafrost. The lakes in the Chumarhe High Plain are saltwater or brine lakes; and in other regions, the lakes contain freshwater or brackish water. Ages of formation, enlargement rates, water and lake-bottom temperatures, the configuration of permafrost, and active-layer thickness were measured at a typical thermokarst lake in the Beiluhe basin between 2007 and 2009. The lake formed about 890 years ago and is 150 m long, 100 m wide, with a water depth of 2.5 m now. The size of the lake is growing at a rate of about 1 m/a at some shores. The lakeshores are underlain by permafrost, which terminates almost vertically at their edge. The mean annual temperature measured at lake bottom in the centre was 5.45 °C, while the temperature of permafrost at 15 m depth in the lakeshore varied from − 0.84 °C near the edge of the lake to − 1.26 °C at 80 m from the edge. The corresponding active layer depths varied from 2.45 to 1.80 m. The configuration of the talik indicates that no permafrost is found beneath the centre of the lake.