Ah - Travertine Dams. Cool formations. The problem with a 'normal' waterfall for your purposes is that erosion would tend to turn a broad fall into a few narrow ones. Travertine dams are formed due to *deposition* of a dissolved mineral - typically calcium carbonate. So wherever water is flowing gets built up instead of worn down. Large gaps where there's too much water flowing to get much deposition are naturally the places where debris tends to go, and whatever gets stuck (logs, leaves, whatever) will slow the flow, improve the deposition, and 'heal' the rift. Google for images of them. You'll see a number of Earthly examples that are not twenty miles wide, but do span hundreds and hundreds of yards. If you don't need *continual* flow across the *whole* width of your 20-mile waterfall, this could be the situation you need. Travertine dams have the nice characteristic that they bow outward, instead of the inward-curving collapse characteristics of most waterfall rims.
To get at least occasional periods of spectacular wide-rim flow, just postulate flooding -- enough of a storm upstream, and the volume of flow goes way up, turning a trickle into a band of falling water.
Iguazu Falls spans 1.7 miles, but that's not continuous. Victoria Falls is "only" 5600 ft wide, but that's a continuous sheet of water... at least in wet season. In dry season it dwindles to a series of smaller cascades separated by rock outcrops. Niagara has a bigger mean annual flow (85,000 cu ft/ sec) but both Iguazu and Victoria have peaked at over 400,000 cu ft/ sec for short periods. Comparison : average flow of the Amazon and Mississippi are about 7.4 million, and 590 thousand cu ft/ sec. Peak for those is more like 10.6 million and 3 million cu ft/ sec, respectively.
Thus a carefully placed waterfall :-) on even an earthly large river could have a flow rate of a hundred to a few hundred times that of Earth's largest waterfalls. So in theory, you could easily get enough water to "look like Niagara" across a twenty-mile span. All you have to do ( ALL :-) ) is devise a hard enough, level enough lip to not wear away in a year or a century, OR devise a way to get a WHALE of a lot of calcium dissolved. For B, how about if your river gathers its water way upstream in rainy mountains, and then flows across a former seabed - like the salt flats in the US southwest. Rivers like the Nile do that - long stretches are in arid surroundings. Say you once had an inland sea, with evaporation jacking up the mineral content of the water. The area shifts to be more arid, and the sea dries up. There's your salt flats. And what can be more level than a former seabed filled over eons with minerals? Nothing the least bit implausible there. Now, do a little creative faulting, and create you a crack across said salt flat. Not just a slim crack - make it like the Great Rift Valley. Heck - imagine that such rifting is what drained the inland sea in the first place -- say, a low-ish part of the former rim-of-watershed is breached - now it's no longer an endorheic basin. Now gather you a whole bunch of rainfall - make it a climate shift and the arid valley is no longer arid, or make it periodic incursions of rain into an otherwise dry zone - take your pick, based on your storyline needs and your wild whims :-).
Boom. Waterfall of whatever width you desire; choose travertine process for keep-the-edge-built-up, or posit a nice layer of basalt as the rim for long-term resistance to erosion. Dial the rainfall up or down for duration of full flow vs trickle. Since you're drawing up the whole landscape, pick a larger or smaller drainage basin to collect rain, and slide it north-south to get the desired wet or dry climate. Want some spectacular differences in normal vs. flood, and all you have to do is place this near a coast that's subject to hurricanes -- several times a year you can have immense flow, and the rest of the time some stable amount.
Such a formation might be short-lived, in geological terms. So what? Cities, highways, and nations are all short-lived in geological terms....
If we're doing rationalization exercises here, don't get me started on the effects of a high concentration of industrious beavers....
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Originally Posted by mozltovcoktail
Like I said in my post it was just the idea that first struck me. Your story, your world , your map, so do it the way you want to. 
