North American cities are full of invisible rivers. A river, in a city, is a long and narrow strip you can't cross on foot, except on a few very precise places, bridges; rivers are drawn in blue on a map. An invisible river is the exact same thing, but it's not written in any way on the map. For someone who travels mostly using the general orientation between the starting and ending points, and sometimes a couple in-between, it is quite unfortunate.