Trying to think of something like this and the only thought I get is Niagra Falls, specifically Horseshoe Falls. They are about 2,600 feet (790 m) wide. You can read about them Here but it says "the Niagara River draining Lake Erie into Lake Ontario, is the collective name for the Horseshoe Falls and the adjacent American Falls along with the comparatively small Bridal Veil Falls, which combined form the highest flow rate of any waterfall in the world."

Obviously that's nowhere near 10-20 miles wide but if it's fantasy and you want to go big well, you just need to make up some reason.