6 of 10 Glacial Lakes in Indus Basin in Himachal, each with a size of 10 ha, are expanding, CWC informs NGT

Among 10 GLs, the size of four lakes ranged between 60 and 70 hectares (Category 2, i.e., S > 60–70), four were between 50 and 60 hectares (Category 3, S > 50–60), and two were smaller than 50 hectares (Category 4, S ≤ 50). The CWC said it monitors GLs larger than 10 hectares in size only.

The six expanding glacial lakes in Himachal are among 34 such lakes identified in the Himalayan region, including Uttarakhand (1 GL), Ladakh (4 GLs), Jammu & Kashmir (5 GLs), Sikkim (15 GLs), and Arunachal Pradesh (3 GLs).

Meanwhile, 19 hydroelectric projects built in the state—mostly downstream on the 23 dams along the flow paths of these lakes — have been studied under the Glacial Lake Outburst Flood (GLOF) risk assessment between 2009 and 2024.

The affidavit was filed in response to a 2024 study conducted by the Himachal Pradesh Council for Science, Technology and Environment (HIMCOSTE), which reported the formation of around 500 new GLs between 2019 and 2023, bringing the total number of GLs in the Indus and Sutlej basins to 1,048. Notably, the Wadia Institute of Himalayan Geology (WIHG) has also prepared glacial lake inventories for Uttarakhand (2015) and Himachal Pradesh (2018), identifying 1,266 lakes with a total area of 7.6 km² in Uttarakhand and 958 lakes with a total area of 9.6 km² in Himachal Pradesh.

The tribunal was further informed that the Triloki Glacier in the Bhaga River Basin, a tributary of the Chenab in Lahaul & Spiti, has been identified for long-term monitoring. For the same, a meteorological (AWS) and hydrological (AWLR) observatory was established near Yotse village last year. Three major glaciers — Triloki, Mulkila and one unnamed glacier — contribute to the discharge measured at this observatory. Geodetic mass balance studies, snow cover assessments, and seasonal melt analysis will be conducted to understand temporal changes and the contribution of snowmelt, glacier melt, and rainfall to river discharge in the basin. In situ measurements of glacier melt will also be taken to validate the results of geodetic mass balance studies. These observations will support validation of glaciological and hydrological models such as HIGTHIM, VOLTA, GLabTop2, SNOWMOD, SPHY, and VIC.

The CWC further informed the tribunal that it has finalised the Criteria for Risk Indexing of Glacial Lakes, providing a structured approach for identifying and ranking lakes based on their likelihood of failure and potential downstream damage in the event of a GLOF. This methodology considers key parameters such as lake size, rate of expansion, slope stability, proximity to other lakes and downstream vulnerabilities such as human settlements and infrastructure, including dams and bridges.

Currently, the CWC monitors 902 Glacial Lakes and Water Bodies (GL&WBs) larger than 10 hectares, based on the Glacial Lake Inventory 2011 prepared by the National Remote Sensing Centre (NRSC). These lakes span the Indian Himalayan Region, covering Himachal Pradesh, Uttarakhand, Jammu & Kashmir, Ladakh, Sikkim and Arunachal Pradesh, and are observed using remote sensing techniques from June to October each year.