Gangotri glacier is the outlet of one of the largest glacier systems in the Himalayas, and the source of the Bhagirathi river, one of the major tributaries of the Ganges. Because of this, Gangotri is often said to be the source of the Ganges. Gangotri terminates at a place called Goumukh, meaning 'cow's head', because the cave at the terminus of the glacier is said to resemble a cow's mouth. At one point the terminus to Gangotri was actually at the town of Gangotri, but the glacier has slowly been retreating, and is now about twenty kilometers away from the town.
Gangotri glacier is nearly thirty kilometers long, and ranges from 0.5 to 2.5 kilometers wide. It has more than 20 tributary glaciers feeding into it, the most important being Raktvarn glacier, Chaturangi glacier, and Kirti glacier. Each of these glaciers, and some of the smaller ones, are still big enough to have their own tributary glaciers feeding into them. Despite the immensity of the glacier, the terminus is not very impressive, being a rubble-covered hill of dirty ice surrounded by mounds of rocks.
Even so, Goumukh and the surrounding area is a Tapovan, and many pilgrims come to bathe in the headwaters of the Ganges. Most of these pilgrims still stop at the town of Gangotri, where the water is still ice cold and there is food and lodging. Some go all the way up to the glacier, although this final trek is taken mostly by sages, sadhus, and tourists.
The Gangotri and Global Warming
The Gangotri has been retreating since the Pleistocene era, when at its peak it was 40 kilometers further down the valley than it is now. It has retreated almost 300 meters (nearly a third of a kilometer) in the last 25 years, which is a very significant increase in rate. If you are at this node, chances are it's because you heard about the glacier in relation to global warming. This makes sense; if India were to loose the waters provided by the melting glacier over a billion people would suffer from the resulting water shortage, and many would die. Those that did not die would have to get their water from other, already overtaxed, water sources.
But the Gangotri is not the best example of the damage is being caused by global warming. While the Gangotri is melting at an increased speed (both at its terminus and on it surface where lakes are forming and growing on top of the ice), the rate of increase in melting is not likely to make it disappear by 2030, as was put out over the AP wire in 2007. The IPCC Fourth Assessment Report does warn that melting glaciers are a big problem, and most of their projections are based around the year 2030, but that's all. Nonetheless, The Washington Post, the Boston Globe, and the BBC, among others, reported that the Ganges might dry up in about 20 years... In all likelyhood, the outflow from Gangotri will be flowing quite steadily at that point, as rising temperatures release tons of ice that had been trapped for millennia.
At some point, the runoff from Gangotri will slow and then stop completely, and if Indians want to drink (and eat) they will have to build a quite extensive system of dams to keep monsoon waters in the mountains, to be released in the dry season. This will be very expensive, and it'll be pity to pay so much to achieve what the glaciers were doing for free, but it is not likely to result in the end of the world, or even India, or even, for that mater, the Ganges.