The Rishikesh rafting season runs from late September to mid June, and it is not one season but two. Post-monsoon, from late September into November, the Ganga is heavy, cold and fast — big waves, fewer boats, and the clearest mountain air of the year. Spring, from March to May, brings warmer water, longer days and the friendliest conditions for first-timers.
December and January are perfectly runnable if you can handle cold water: flows are low, rapids are technical rather than huge, and the beaches are empty. February sits in between, and by late May the heat in the valley makes an early departure essential.
Rafting closes through the monsoon, roughly late June to mid September. That is not a commercial decision — flows on the Ganga in July and August are genuinely unsafe and operations are suspended on official orders.
If you want maximum adrenaline, come in October and book Kaudiyala. If you are bringing family or nervous friends, come in March or April and start with Shivpuri or Brahmpuri.