[Hussell]

> Real rivers go somewhere. They don't just stop in the middle of nowhere.

Yeah? Have a look at the Morghab River in southern Turkmenistan. It flows from some mountains in Afghanistan out into a desert, where it splits up into a delta and just sort of... stops. The satellite view on Google Maps is striking. (There are several canals connecting to the delta now, but historically the river stopped in the middle of nowhere.)

Rules of thumb like this aren't laws, just patterns which have exceptions. (That said, yes, imaginary maps are sometimes ridiculous.)