Even a magical world needs to follow player/reader expectations, and that means you will generally want to stick to the laws of nature as we know them. If you add weird physics, you need to deal with the consequences. For example, take Korash's idea. Of course it would work. But you'll need to explain it. Why are they fighting? Earth and water are not tarditionally opposed to oneanother. What happens in places where there's no water to keep the earth in check? Does the Earth push up without opposition, resulting in huge mountains? What about erosion? The huge amounts of water would carry a lot of earth downriver, where it would create a gigantic delta/marsh/whatever. And then there's the question - if water and earth fight here, what about the other elements?

Of course you can just handwave all of that away, but that's cheating.

Instead, I'd think about the effect I want to achieve. What purpose does my 20mile-wide waterfall serve? Is it just scenery? Do I need it for a climactic scene in a book? Etc. Then try to come up with something else that works. Limiting oneself isn't always a bad thing, it can result in some great work. (Compare old star wars, which had to work under physical and, at first, budget limitations, to the new star wars, where they had all the technology and all the money they could hope for...)