# Hazard Assessment for a Glacier Lake Outburst Flood in the Mo Chu River Basin, Bhutan
## 2 Study Site  
The Kingdom of Bhutan is a buffer state between China and India in the Eastern Himalayas. Dominated by the south-western monsoon, annual precipitation is high (1000-2500 mm) and the maximum occurs in June to September [19]. This leads to the formation of summer accumulation type glaciers in high altitudes. The total glacier cover of 838 km\({}^{2}\) around 1980 has been reduced by 23.3% to 642 km\({}^{2}\) in 2010 [20].  
The Mo Chu catchment is located in the Lunana region of North Bhutan, stretching from above 6000 m a.s.l. at the highest peaks along the Chinese border (Masang Kang: 6710 m a.s.l.) to 1220 m a.s.l. at the confluence of Mo Chu with Pho Chu River at Punakha Dzong (Figure 1), an administrative center and major tourist attraction. From here, the river is named Punatsangchu in Bhutan and Sankosh later in India, where it eventually meets the Brahmaputra River.  
In the Mo Chu catchment, the glacier area decrease since 1980 was 25.5%, leaving 195 glaciers with an extent of 114 km\({}^{2}\) in 2010 [20]. Currently, a total of 66 glacial lakes exists in the catchment [21].  
Two PDGLs were identified by the NCHM [13], \"Mo_gl235\" and Sintaphu Tsho (tsho means \"lake\"), both are located northeast of Gasa village. Since \"Mo_gl235\" is the smaller lake and has not been surveyed bathymrically up to now (Phuntsho Tshering, pers. comm., 12. March 2021), we focus on Sintaphu Tsho. This moraine-dammed glacial lake is located at an altitude of 4480 m a.s.l., has an area of 238,314 m\({}^{2}\), and a volume of 6.2 mio. m\({}^{3}\)[13]. A second lake is located at a distance of 100 m up the valley. While the NCHM [13] mentions a growing supraglacial lake in close proximity, the high-resolution WorldView-2 imagery suggests that the active glacier is located in a horizontal distance of 250 m from the second lake. A supraglacial lake cannot be identified on the recent image. The upper lake is smaller (ca. 70,000 m\({}^{2}\)) and seems to be shallower, because the areal  
Figure 1: Map of the study site and satellite image of the lake surroundings (location indicated by black frame). Data sources: AW3D30 (elevation), RGI 2017 (glacier extent), World Imagery by Esri, Maxar, Earthstar Geographics, USDA FSA, USGS, Aerogrid, IGN, IGP, and the GIS User Community (satellite image).  
extent changes significantly with the varying water level on different Google Earth images. For this reason, we exclude it from our GLOF study.  
For Sintaphu Tsho, the lake size and the short distance of the glacier justify the classification as a PDGL. The lake is located ca. 85 km upstream of the confluence with Pho Chu near Punakha, which is also the first and the largest settlement (population: 21,500 according to worldpopulationreview.com) along the route of a potential GLOF. Two more villages follow downstream 22 km and 36 km after the confluence, whereby two large hydropower projects (Punatsangchhu I and II) are under construction, which involve a 137-m-high and 279-m-wide concrete diversion dam across the river, respectively. Although the power houses themselves are underground, the projects might be vulnerable to large GLOFs.