I heard a few years ago that geographers had reckoned that the Amazon is longer than the Nile–about 4,200 miles. This doesn’t make sense! I have a “mileage wheel” I usually use on street maps to calculate the distance I’ll have to drive to get somewhere. But I used this on an atlas map of South America, and the length of the river–starting at its delta around Ilha do Marajó and going upstream–seems to be only two thousand miles or so. Once you cross the Brazil-Peru border, moving upstream, the Amazon splits into three or four tributaries, none carrying the name “Amazon.” Have I missed something here? 
it’s about an inch and a half long in my atlas.
I think it’s about 4 million years long, since the last shift of the Aswan Gap.
Oops. I misread and thought you meant the Nile.
The Amazon has been there much longer, since Pangaea.
One other thing to note about measuring along a map: a lot of bends and oxbows are going to be straightened out, the smaller the scale of the map.
Ah–but which one of those tributaries issuing from the Peruvian Andes is used in recoking the Amazon’s length? My Hammond Atlas has no clues…
Abraham Lincoln said that it was long enough to reach the ocean.