I really think that Ganga is major part of Indian beliefs. Ganga is holy river for Indians where lots of Indians come to wash away their sins. There are many India holy books that mentions the importance of Ganga. Since India being religious country it is a really big part of their life.
Although Ganga is an important landmark and considered holy in Indian Culture, to say that a river could ever be the backbone of a culture ignores the diverse population that makes up the culture. The backbone of every culture is the people within it, Ganga would be there with or without the local population and even those that move away from Ganga continue to retain their cultural values and beliefs. It is those beliefs and values and the people that follow the traditions that are the real cultural backbone.