The Gangotri glacier at the base of Bhagirathi peaks I, II, and III, is the source of the Bhagirathi River which is an important tributary of the great Indian River Ganges. The glacier is about 30 kilometres long (19 miles) and 2 to 4 km (1 to 2 mi) wide, is one of the largest in the Himalayas. This glacier has been constantly receding since measurements began in 1780. Data for 61 years (1936â€“96) show that the total recession of Gangotri glacier is 1147 m, with the average rate of 19 m per year. However, over the last 25 years of the 20th century it has retreated more than 850 meters (34 meters per year), and 76 meters between 1996 and 1999 (25 meters per year).