http://www.shadedrelief.com/world/Data/map1_type.jpg a good map of mountain ranges to search for counterexamples. Lake Maracaibo in Venezuela is surrounded by mountains on three sides and the Magdalena river runs parallel with the Andes and has some squarish curves. It also seems likely that the map could be thought of as not being accurate itself with the "real world" geography of Middle Earth given its sketchy design.