For the first time, geologists in India have been able to map the entire Subansiri river basin irrespective of the geopolitical boundaries which divide India and China. Data showed that 30 percent of the glaciers are retreating rapidly in this part of the Eastern Himalaya. According to the authors of the study, the data-set can be the reference for future research on the transboundary river basins especially at the time when the Indian and Chinese governments are building mega dams across the region.
This work was initiated by Prof. Sarat Phukan, and Dr. Swapnamita Choudhury Vaideswaran, which led to a collaboration between the Wadia Institute of Himalayan Geology and Gauhati University. The team used freely-available Landsat satellite data from the United States Geological Survey (USGS) to prepare an exhaustive inventory of glaciers and glacial lakes in the Subansiri basin spread over Arunachal Pradesh and Tibet.
Unlike the Western Himalayas, which have the Ganga, Yamuna or the Indus basins, there is a dearth of data on the transboundary river basins especially in the upper reaches of the entire Eastern Himalayas. The researchers also estimated the ice reserve in the Subansiri basin and found that 30 percent of glaciers are receding due to climate change. However, some glaciers also seem to be quasi-stagnant. The researchers also estimated the change in size of the pro-glacial lake of the Daisaphu Glacier over 35 years, thereby increasing in area water volume over the years. This is especially important data to monitor for sudden glacial lake outburst floods in the region. The results also, for the first time, bring out an estimate of ice reserves in the 37 valley glaciers in the Subansiri Basin. The study was published in the Geological Journal of Wiley on January 21. EastMojo recently spoke to Swapnamita Choudhury and Sarat Phukan on their findings. Excerpt from the interview:
Can you explain to us the significance of your research work on the Subansiri basin? What prompted you to study the same and prepare a glacier inventory?
This is for the first time that an authenticated, updated, complete inventory of glaciers and glacier lakes in the entire Subansiri Basin on a sub-basinal level using freely-available Landsat satellite data has been constructed. We report 390 glaciers and 52 glacier lakes in the basin. The most significant output from this work is estimation of the ice reserve amounting to around 11.95 cubic kilometres in the valley glaciers in this basin. Further, we have also calculated the recession of the valley glaciers which range between 2 and 51 meters per annum (ma-1), where, 30% of glaciers have retreated at a rate of less than 10 ma-1, 55% glaciers have retreated at a rate of >10 ma-1 and 15% glaciers show no frontal changes since 1985.
The Subansiri River is one of the most important tributaries of the Brahmaputra River, contributing about 7.92% of the total river flow. This river basin is especially significant with the construction of the 2,000 MW hydropower project, downstream at Gerukamukh in Assam. However, if we think about the water reserve in the glaciers of this basin, there has been insignificant data in the form of an inventory for the entire basin. This is because of its difficult terrain; the problem being further intensified due to the distribution of the snow-covered regions between India and Tibet. Some inventories existed, which either did not cover the entire basin, or had not been updated. Some inventories covered only the Indian part, while some compiled the inventory of glacier lakes only, that too within the Indian territory, while most of the glacierized regions lie across the country. Therefore, the need for an updated, complete inventory of glaciers and glacier lakes was considered utmost necessary. This inventory will help in not only understanding the future water scenario in the downstream region, which is dependent on the water reserve across the country, but also help in mitigation and management of mountain hazards, which can create havoc downstream. We have seen quite a few devastating scenarios in the Kedarnath disaster of 2013, and then again, the Rishi Ganga flash flood in 2021.
What kind of challenges did you face while conducting your research work? Could you also explain the time that you took and the rigorous processes involved in the study?
We used Landsat remote sensing data for construction of the glaciers and glacier lakes inventory. Initially, it was surprising to find a lack of available satellite data through different free data portals for the Eastern Himalayas. The latest satellite data we received was for the year 2016. Whereas, free, good resolution free data was available over the years for the Western Himalayas. Upon scouring through literature, we saw no substantial work has been done in the glacierized regions in Eastern Himalayas. The studies in Western Himalayas are also corroborated with ground work, due to the accessibility in terms of terrain and geopolitical location.
The Survey of India maps covered the Indian part only, and most of the maps were classified as the region is strategically located. Further, for ice volume calculations a lot of considerations had to be put for the correct use of algorithms, as this was done for the first time in this part of the Himalaya. References in literature had to be validated before being reported. We are thankful to the anonymous reviewers, and especially to the Editor-in-Chief of the Geological Journal, who painstakingly helped in editing our manuscript with unbiased review.
Upon submission of this manuscript, the Journal was very prompt in the review process and we could bring out the work on time.
What are deeper ramifications of your findings?
Our work is the first assessment of the ice reserves in the Subansiri Basin. It is also a comprehensive, updated and validated inventory of the number of glaciers, and glacier lakes in the basin. This inventory can be the reference to several studies from here on, be it monitoring of mountain glaciers in Eastern Himalayas, or climate change scenarios now, or in future. Our report gives the recession rates of the valley glaciers in the basin. It is alarming to note that some glaciers show greater than 50 ma-1 rate of recession. But also, some are quasi-stagnant. So, it will be interesting to monitor from here on, how the glaciers will behave in future. Enhanced rate of recession has been reported by us after 2009, which also corresponds to rising temperatures as seen from 100 years of climate data analyzed by us.
This inventory also stands as a base map for glacier lakes. The lakes are not big enough to pose any impending danger for outburst floods. However, continuous monitoring of the lakes will give a picture of how the lakes are changing in size from here on.
While mountain disasters should not be causing much harm in the Indian regions, downstream in the Arunachal and the Assam valley as it is very far from the steep slopes and the glacier lakes. However, the Tibet region, where most of the glaciers lie, is often affected by soil slides. The Tibetan part of the Himalaya is actually the leeward side of the Himalaya, which is a rain shadow region. Therefore, the steep terrain is marked by loose, eroded sedimentary deposits, or what is called the Loess deposits. This loose material often slips and falls often on to the upstream tributaries of the Brahmaputra above Arunachal Pradesh. Such slips can induce floods, but downstream in India we only observe silty-muddy water at times.
What kind of support did you get from state, non-state as well as inter-state agencies in this research work? What more could be done in this field?
Our study was based on freely available satellite data. Therefore, we did not seek any support from any funding agency. The support from the Institutions and Universities where the authors belong to, definitely provided necessary facilities like software/infrastructure.
Regular monitoring in future years is necessary for the valley glaciers in this basin. We have calculated recession rates between 2 and 51 ma-1 by the year 2016. We observed an enhanced retreat of 21 ma-1 for some of the glaciers beginning from 2009. While it is understood that north-facing glaciers show slow retreat rates, we have observed quite a few north-facing glaciers in this basin to show very high retreat, up to 51 ma-1. We have correlated the enhanced retreat to enhanced temperature conditions post 2009. Therefore, to understand the enhanced effects of climate change in this basin, regular monitoring and updating of this inventory will be beneficial.
The Lower Subansiri dam is going to be commissioned later this year. In that context, your study is really timely. Is your study influenced by the dam-building exercise in the Subansiri river?
The intention of doing this study was the dam because when there is a big dam coming up downstream, we should have a proper assessment of what’s happening upstream. This basin, in particular, has an upstream glacierized region in Tibet. So, a complete assessment of the basin needs to be done in context of what’s happening in the entire basin, and how we might be affected if anything happens like a glacier lake outburst flood (GLOF). There is also a new thing now which is known as landslide lake outburst flood (LLOF). We have got two examples: the Kedarnath disaster and Rishi Ganga floods. Kedarnath caused havoc. So how did the 2013 Kedarnath disaster get triggered? It started with the Chorabari Glacier lake. Wadia Institute of Himalayan Geology has been working on the Chorabari Glacier for several years now. We have a permanent station there equipped to monitor weather etc. There is a pro-glacial lake in Chorabari, like the lake in the Subansiri glacier we have reported in our study, called Daisaphu Glacier Lake. There is a steep mountain wall next to the lake near the Chorabari Glacier. We have taken measurements of the depth of water in the lake. However, we noticed when there is a high load of snow on the wall, the snow would slide down the wall and settle around the lake. But in 2013, the Kedarnath disaster was triggered by an avalanche right on the lake and huge and solid chunks of ice fell on the lake which created a kind of tsunami. We are talking about a region where the slope angle is very high. Therefore, when something huge comes down, you can only imagine the speed at which it would fall. On top of that, there was this heavy and continuous rainfall. I remember I was in Dehradun and it rained for three days continuously. It was like someone was emptying a bucket of water. Due to highly intense rainfall, this disaster happened. Rishi Ganga is also similar to the Kedarnath disaster. Rishi Ganga had two hydroelectric power projects. One hydroelectric power project was a smaller one where Rishi Ganga meets Dhauli Ganga. There is another one downstream which is known as Tapovan project near Joshimath. During this disaster, a landslide happened on the glacier that feeds the Rishi Ganga River. When such highly intense landslides happen, there is generation of heat as well. That broke a chunk of the glacier and melted the water. All this water gushed down and completely destroyed the smaller hydroelectric project in Rishi Ganga and impacted the Tapovan hydroelectric power project. But one has to take note that these projects and the impacted areas are not very far from where these events occurred on the glacier. However, in our study area, the Subansiri river basin, the upstream region winds around Tibet for a very long time. It is a very long distance, roughly around 100 kilometres. Rishi Ganga was a landslide on the glacier and the Kedarnath was kind of a GLOF but they were closer to the area that was affected. In our study area, in Subansiri, we expect that there won’t be any major damage downstream in India because this upstream region is too far. It is highly unlikely that the events in upstream of Subansiri basin would impact the dams. There is, however, a need to continuously monitor these lakes on the upstream regions of Eastern Himalaya. There are around 52 lakes in the Subansiri basin. We have studied only one lake in great detail. How these lakes are increasing in size is something we need to observe and monitor continuously.
What about recession of the glaciers in your study area? You are saying that 30 per cent of the glaciers you studied have receded. Would that impact water availability in the Northeastern region?
Yes, definitely that would impact. How these glaciers are changing, how they are receding and the accelerated recession in the current years definitely need to be taken into account. But till now we had no data. We have experienced the recession of the valley glaciers. There are different types of glaciers. We have prepared the inventory of the glaciers and the glacier lakes. We have just studied one basin. We plan to extend the study to the other basins too to understand the complete water availability scenario in the Brahmaputra valley.
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