Subject: Approximantly how many gallons does it take to fill the grand canyon

I am asking this for my 5 years old son and i want to give him a right answer . 
I have to figure out how many gallons it takes to fill the entire grand canyon. 
i know that it is 10 cubic miles. I also know that there is about 250 gallons 
in a cubic meter. i cannot figure out how to figure out how many cubic meters 
in a cubic mile. i have searched and searched is there such a thing ?.I hope 
you can help me. thank you

Re: Approximantly how many gallons does it take to fill the grand canyon