More than the snow, think of the water.
If my memory is correct, the volume of a cylinder is pi times radius squared times height, so:
V=pi x 2000ft^2 x 900ft
V = 11,304,000,000ft^3 (That's 11.304 BILLION cubic feet of snow).
Google tells me you can expect at least 144 cubic inches of water per Cubic foot of snow.
144 x 11,304,000,000 = 1,627,776,000,000in^3
231 cubic inches of water in a gallon means:
Pretty much no less than 7,046,649,351 gallons of water will be released as it melts(Assuming I haven't fucked up my math completely)