First we find the volume of a rectangular solid 1 mile by 1 mile by 1 inch.

V = lwh = (1 mile)(1 mile)(1 inch)

We change each of the (1 mile)s to (5280 feet)
We change the (1 inch) to ( feet)

V = lwh = (5280 feet)(5280 feet)(feet)

V = 2323200 cubic feet of water

Now we look in a reference book and find that

I cubic foot of water weighs 62.38 pounds

So we multiply 2323200 by 62.38 and get 144921216 pounds.

Now to change that to tons we divide by 2000 and get

72460.608 tons, or we'd just say about 72,500 tons.

Edwin