We were told that volume of water that flows over the fall each second is 2.9 x 10^5 gallons.
Recall, 1 hour = 3600 seconds
If 1 second = 2.9 x 10^5 gallons, then
3600 seconds = 3600 x 2.9 x 10^5
= 1.044 x 10^9 gallons
Thus, 1.044 x 10^9 gallons of water will flow over the falls in an hour.