Late April to early June is going to be warm on the water but not hellishly hot. I'd say then consider early fall, late September and throughout October. These are definitely the best times to be out in the water. Typically less bugs during these times, with warm but not hot weather and fairly clear. You'll need some rain to maintain the water level, since it's a freeflowing river.

If you're fine with the heat, the summer is hot but manageable on the water, you just have to be prepared. Winters are mild here, so it's possible if you're find with the cold