The glaciers of Bhutan, which covered about 10 percent of the total surface area in the 1980s, are an important renewable source of water for Bhutan‘s rivers. Fed by fresh snow each winter and slow melting in the summer, the glaciers bring millions of litres of fresh water to Bhutan and downriver areas each year. Glacial melt also adds to monsoon-swollen rivers which may be a contributing factor to flooding. Where glacial movement temporary blocks riverflows, downstream areas may be threatened by glacial lake outburst flood (“GLOFs”). Although GLOFs are not a new phenomenon in Bhutan, their frequency has risen in the past three decades. Significant GLOFs occurred in 1957, 1960, 1968 and 1994, devastating lives and property downstream. According to the Bhutan Department of Energy however, the majority of rivers in Bhutan are more susceptible to fluctuation with changing rainfall patterns than to flooding directly attributable to glacier or snow melt.
Bhutanese territory contains some 677 glaciers and 2,674 glacial lakes and subsidiary lakes, out of which 25 pose a risk of GLOFs. The vast number of glaciers in Bhutan are classed as “valley” and “mountain glaciers,” although significant numbers of “ice apron,” and “niche glacier” types also exist.:F323 Some glacial lakes, such as Thorthormi Lake in Lunana Gewog, are not a single bodies of water but collections supraglacial ponds. Most glacial lakes identified as potentially dangerous feed into the Manas River and Puna Tsang (Sankosh) River water systems of north-central Bhutan. During a GLOF, residents of nearby downstream villages may have as little as twenty minutes to evacuate; floodwaters from one 1994 GLOF at Luggye lake took about seven hours to reach Punakha, some 90 kilometres (56 mi) downstream.
For public safety, these glaciers and glacial lakes are maintained by the Ministry of Economic Affairs’ Department of Geology and Mines, an executive (cabinet) agency of the government of Bhutan. The Department, as part of its environmental “mitigation projects,” aims to lower the levels of glacial lakes and thereby avert GLOF-related disaster. In 2001, for example, scientists identified Lake Thorthormi as one that threatened imminent and catastrophic collapse. The situation was eventually relieved by carving a water channel from the lip of the lake to relieve water pressure.  The Department uses silent explosives and other means it considers environmentally friendly in order to minimize the ecological impact of its mitigation projects. These projects, however, remain difficult to conduct because of the weather, terrain, and relative lack of oxygen at the glacial lakes’ altitudes. As of September 2010, GLOF early warning systems were slated for installation by mid-2011 in Punakha and Wangdue Phodrang Districts at a cost of USD4.2 million.