bhagirathi glacier retreat – From a Glaciers Perspective

Gangotri Glacier snowline averaging 5600 m on Oct. 16, 2020 in Landsat image, magenta dots indicate the snowline. Yellow line is the Randolph Glacier Inventory margin. Tributaries: K=Kirti; G=Ghanohim, Su=Sumeru, M=Maiandi, S=Swachhand.

Gangotri Glacier, India is in the Bhagirathi River watershed and is the largest glacier in the Garhwal Range of the Himalaya. Gangotri Glacier supports hydropower as its meltwater runoff passes through three hydropower plants generating 1430 Megawatts including the Tehri Dam, Maneri Bhali I and II. From 1968-2006 the glacier retreated 800 meters, ~20 meters/year (Bhambri et al 2011). The glacier has continued to thin and tributary inflow has declined with a tributary (Chaturangi) separating during this period . Bhambri et al (2011) noted that recession rates have in the region have increased since 1990. Bhattachaya et al (2016) expanded on this work noting that the velocity of Gangotri Glacier declined during 2006-2014 by 6.7% from 1993-2006, indicating a reduced volume of accumulation flowing downglacier. They also noted an increase in the rate of debris-covered area expansion on the main trunk of Gangotri Glacier from 2006-2015, indicative of an expanding ablation zone. Bhattachaya et al (2016) report a retreat rate of 9 m/year 2006-2015, which is less than before, but the down-wasting in the same period 2006-2015 was higher than during 1968-2006.

Gangotri Glacier boundary and flow directions on Digital Globe image with GLIMS glacier outline indicated.

Gangotri Glacier is a summer accumulation glacier with the peak ablation period low on the glacier coinciding with peak snowfall high on the glacier during the summer monsoon. In the post monsoon period of October and November precipitation is low and melt rates decline. Kundu et al (2015) noted that from Sept. 2012 to January 2013 the snowline elevation varied from 5080 m to 5174 m . This contrasts to an ELA of 4875 m reported by (Bhattachaya et al 2016) and 5100 m (Bhushan et al 2017).

On October 9, 2016 the snowline was at 4850 m on the main trunk and on the tributary Ghanohim Glacier, and 4750 m on the tributary Kirti Glacier. By November 30th a Landsat image indicates the snowline has risen to 5400 m on the main trunk and Ghanohim, the snowline is at 5800-5900 m on the glaciers in the Swachhand tributary valley, at 5600 m on Maiandi Glacier and 5700 m on the last tributary entering from the north. A Landsat image from Dec. 9th indicate that the snowline remains approximately the same as on Nov. 30th. The substantial post monsoon snowline rise in 2016 is illustrated in this article on Gangotri Glacier.

**Gangotri Glacier snowline purple dots in Landsat 11-30-16 image, red arrow is terminus.**

On October 16, 2020 a warm/dry post monsoon season has led to the snowline rising rapidly on Gangorti Glacier. On Kirti the snowline is at 5500 m, on Ghanohim at 5400 m, on Sumeru at 5500 m, on the main stem 5600-5700 m, on Maiandi at 5700 m and on Swachhand at 5800 m. The snowline is averages 5600 m the same as in 2016. Below the snowline there is limited older firn exposed, indicating that limited snow is retained below 5400 m on Gangotri Glacier from year to year. The bare glacier ice has a lower albedo then snow increasing melt and further reducing glacier mass balance.

The accumulation area ratio is the percentage of a glacier in the accumulation zone and is typically above 50% for a glacier in equilibrium. On Gangotri Glacier in November 30, 2016 the accumulation area ratio was 20-24%, indicating a large mass balance deficit. On October 16, 2020 the AAR of Gangotri Glacier was 23%. More important the firn line indicates that the AAR is now consistently at ~25%. This will drive continued retreat and will accelerate the retreat. How far upglacier on Gangotri Glacier do you have to travel to reach the snowline? On the central flow line 29-30 km along the 31-33 km length. The increase in temperature has led to a tendency for snowlines to rise in the post monsoon period and remain high into the winter season on other Himalayan glaciers such as West Rongbuk Glacier and around Mount Everest in 2019.

Central flowline for Gangotri Glacier, 32 km long, with 29.5 km in the ablation zone. Landsat image from 10-26-2020.

Jaonli Glacier is in the Pilang basin which feeds the Bhagirathi River in Uttarakhand. The glacier is 20 km west of the well known Gangotri Glacier and 30 km east of Jaundhar Barak. The glacier is one valley north of the well studied Dokriani Glacier. Jaonli has a heavily debris covered terminus which slows the retreat of the actual terminus, while upglacier thinning has been quite rapid. Here we examine Landsat imagery from 2000 to 2012 to illustrate the change due to increased melting. The glacier provides hydropower as it passes three Hydropower plants generating 1430 MW, including the 1000 MW Tehri Dam and reservoir, which also provides flood control, such as this past week of June 17, 2013(second image). The Tehri Reservoir level rose 25 m within 48 hours which is a storage of approximately 1.3 billion cubic meters. Jaonli Glacier is a heavily avalanche fed from the huge wall of rock on its northeastern flank, as noted by light blue arrows. In each image the yellow and orange arrows indicate the same location for comparison of the width of debris cover. The purple and red arrow indicate where two tributaries flow down to meet the Jaonli Glacier.

In 2000 the clean ice region of the main valley tongue of the Jaonli Glacier is 400 m wide at the yellow arrow and 500 m wide at the orange arrow. The eastern tributary at the red arrow joins the main valley glacier. At the purple arrow two small ponds are developing as the tributary that does not reach Jaonli melts back. By 2012 the debris free ice surface is 100 m wide at the yellow arrow and 200 m wide at the orange arrow. The debris cover spreads across the glacier as the clean ice melts faster thinning, and the debris covered area is then on an increasingly wide and high ridge above the clean ice and the debris then falls and oozes down onto the cleaner ice. The tributary at the red arrow still reaches the main glacier but is now covered by debris. The two ponds have merged at the purple arrow to create a larger terminus lake. IN Google Earth the images are from 2011. The first is a cross ice view towards the red arrow tributary, indicating the same locations and the extent of the debris cover. The second image is a closeup of the terminus, with a pink arrow indicating the 2011 terminus, note river issuing here and large ice face. However, there are two other developing termini with lakes forming 1 km upstream of the main terminus, green arrows. This stagnant ice in between will continue to melt and collapse. The glacier has retreated 160 m from 2000 to 2012, but will undergo an additional one kilometer retreat to reach the lake locations. This glacier fits well the overall retreat in the region (Kulkarni et al, 2007).Dokriani Glacier retreated at a rate of 17 m/year from 1962-2000 (Bhambri and Bolch, 2011). In the nearby Tons River Basin Mehta et al (2013) noted the 1962-2010 retreat of three glaciers Jaundhar (34 m/year), Jhajju (15 m/year) and Tilku Glacier (13 m/year).