I have not run any of these rivers, though I am somewhat familiar with the Beas. This information is solely based on third-party opinions.
This river is known for its natural beauty and picturesque setting. It runs along
the Indo-Nepal border and is not a commercially run river, due to its
remoteness. Instead, the operators will organize an expedition for any group
(at least 1 raft). Be prepared for a leisurely trip - you will even have
sometime for fishing (if interested), and sightseeing. Duration of the
expedition is 5 days (excluding road travel).
Upstream from Deoprayag, the Bhagirathi is an exciting and thrilling river. In complete contrast to the Mahakali, this river is a quick succession of big rapids. There are several grade IV rapids, and if you have finished the Ganges, you are ready to graduate to this river (with a good river guide of course). This too is an expedition course, and ends at Kaudiyala after rafting of 4-5 days.
The Zanskar lies in the remote and extremely cold region of Ladakh. Rafting on the river is only possible for a few months during the year. The water is icy cold and fast. This is an exciting river and requires a fairly long expedition. Be ready to take at least 2 weeks off for this trip. The river is graded IV+ though the individual rapids are not graded above III+.
While most of the rivers are located in the newly formed Uttaranchal state, in the Garhwal and Kumaon regions, the Beas lies in the Kullu Valley. It is an extremely fast river with big rapids. Rafting is possible only for a short time because the water level recedes very quickly (due to proximity to the mountains). There are some organizers in Kullu who run rafting courses on the Beas. HP Tourism also organizes such tours.
