Water volume of 35.2 km3 (28,500,000 acre·ft)----- 28.5 million acre-feet x 43,560 square feet in an acre = 1.24 trillion cubic feet x 8.1 (gallons in a cubic foot) = 10 trillion gallons
This question cannot be answered accurately because of the fluctuation in water levels throughout the year , but I believe between a lot , and a much larger amount.