Sikkim used to be Nepal and the rivers for sure have not changed their resemblance. Season and temperatures are the same as in Nepal and most of the rivers are still virgin because of travel restrictions in most areas.
The only really known river is the Tista which, like the Beas has an easy rafting section.
Kayak Himalaya
Here we will focus on more difficult waters as well as some rivers I have discovered and paddled myself.