The glaciers in Bhutan, which covers about 3 percent of the total surface area, are responsible for feeding all rivers of Bhutan except the Amochu and Nyere Amachu.
Not much historical information is available on these glacial systems; the first modern survey was conducted by Augusto Gansser-Biaggi in the 1970s, who developed a chronology of past glaciations, and warned of imminent glacial lake outburst floods (GLOF) in Lunana. This prompted the Geological Survey of India (GSI) and Geological Survey of Bhutan (GSB) to carry out joint surveys in the mid-80s, and their report rejected Biaggi's threat-assessment. On 7 October 1994, a GLOF from Luggye lake (part of Lunana glacial system) led to over 20 fatalities in Punakha.
This disaster, the first of its kind in Modern Bhutan, increased the frequency of glacial-system-surveys by manifold. Evaluation of GLOF hazard has since formed a significant component of glacial expeditions. Multiple inventory-lists of glaciers and glacial lakes in Bhutan exist. The precise figures vary to some extent, based on the methodology used.
Inventories
Glaciers
Multiple trans-Himalayan inventories of variable accuracy and area-span have existed for decades. This section concerns with inventories specifically curated for Bhutan.
The first inventory was compiled in 1996 by Phuntso Norbu (of the Division of Geology and Mines) using satellite data from 1989–90 and topographic map of 1962/63. A revised edition was published in 1999 by GSB. The same year, another inventory was published by China Science Publishing & Media on the basis of near-infrared Landsat 2 imagery (1975–1978) and some aerial snapshots — 649 glaciers were located, covering an area of 1,304 sq. km. and a volume of 150 cubic kilometer. A more accurate inventory, prepared from 1993 SPOT Imagery, Landsat data of 90s, IRS-1D data of 1999, and GSI topographic maps of 1950-60s, was published in 2001 by the International Center for Integrated Mountain Development (ICIMOD). 677 glaciers, covering an area of 1,317 sq. km were located and classified per Mullerian (WGMS) nomenclature.
In 2011, ICIMOD recompiled its data-set for the entire Hindu Kush, using an automated classification of satellite imagery from 2005 ± 3 years in combination with digital elevation model — the area was revised to 642 sq. km (based on Bhutan’s new national boundary). In 2012, data from World Glacier Inventory (WGI), Global Land Ice Measurements from Space (GLIMS), and Natural Earth (NE) were blended and refined to create yet another inventory of glaciers — the area estimates came out to be 1930 sq. km. (did not take the new boundary into account). The ICIMOD database was updated in 2014 (using Landsat imagery from 2010) — no areal difference existed.
In 2018, the National Center for Hydrology and Meteorology published the most recent inventory of glaciers in Bhutan using automatic classification of Sentinel-2 data from 2016 — 700 glaciers were located covering an area of about 629.55 sq. km.
Glacial Lakes
Glaciers
By convention, glaciers are located in the river basins, they lie — Wang Chhu basin (consisting of Pa Chhu, Ha Chhu and Thim Chhu sub basins), Punatshang Chhu basin (consisting of Pho Chhu, Mo Chhu and Dang Chhu sub basins), and Manas Basin (consisting of Mangde Chhu, Chamkhar Chhu, Kuri Chhu, and Drangme Chhu sub-basins). Two other river basins exist in Bhutan corresponding to Amo Chhu and Nyere Ama Chhu but is not associated with any glacier.
In order of area (or volume), the vast majority are classifiable as "valley glacier", and "mountain glacier". "Ice apron", and "niche glacier" types exist in significant numbers but occupy far less area (or volume). Far rarer are "cirque glaciers", and "ice caps". The "valley glaciers" are located along the southern flanks of the main topographic divide of the Bhutan Himalaya, and are characterized by debris-mantled snouts. They are primarily concentrated in the upper part of the Pho Chhu basin and Northern Basin. "Mountain glaciers" are common on peaks, and back walls of valley glaciers. "Cirque glaciers", "ice caps", "ice aprons", and "niche glaciers" are common on plateaus and ridges that stretch to the south from the main Himalayan divide; they are virtually debris-free. Rock glaciers exist in many places along the Snowman Trekking Route in northwestern and northern Bhutan. The south flowing glaciers exhibit steeper head-walls, more debris cover, and slower flow rates than their north-flowing counterparts.