OK, I called River Riders and if we book online we save another $2 per person (since we're going on a Sunday; no discount on Saturdays). But because when you book online you have to pay for it up front, I don't want to pay for 10 people until I know for sure that one more person is going (otherwise the money situation will get too complicated if we stay at 9 people but pay for 10.)

So I'm going to hold off, and maybe F8th or Zen's friend will commit, and then we can firm it up. If we get too close to August 10th and still have 9 people, we'll just lose the discount. River Riders said we need to book in advance, but we have time.