The Gangotri Glacier, one of the largest glaciers in the Himalayas and one of the main sources of water for the Ganges River. The head of the glacier can be seen at lower right. The Gangotri flows around 30 kilometers (18 miles), but like many of the world’s glaciers, it is in retreat. It has been retreating for a long time, and observations show that the glacier is now thinning. With such a large portion of the world’s population dependent on water from these cold heights, changes in the size and flow of these glaciers can bring serious consequences by affecting the amount of water arriving downstream. January 7, 2018.