Every dry season there could be a chance that the dry season is actually a winter season. This would be like a dry season but with more disadvantages. During the season, rivers would freeze, crops would start dying, and the beavers would also get a "warmth" meter. Keeping this full would require either more campfires or "coats" made out of grass fibers (or maybe other animals that the beavers could domesticate?). Irrigation wouldn't help crops either, but you could build greenhouses to keep crops inside warm during the winter.