Not sure about optimal tubing flows but I'd expect you can tube it up until the time your beer is warmer than the water. Average flows don't drop below 600 cfs until about December. Extreme tubers will want a dry suit for that!
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse