Pond is 66 inches wide x 58 inches deep.

Using an average radius of 62 inches and the volume of a cylinder pi*r squared * depth and halving this since we are only half a cylinder, and then multiplying by 5.9 gallons per cu ft, the volume at 13 inches deep is 268 gallons. 15 inches deep is 309 gallons

Here's the calculation scratchpad: