I like Thunder Rift, it's a solid little setting. But seriously, did everyone at TSR flunk Earth Science when they were in school or something? There are all kinds of rivers intersecting at extremely unlikely angles, flowing uphill, and branching off miles from any delta.
I'm not claiming I'm the world's foremost physical geography expert, but I at least know what gravity is-- screw a wizard did it, rivers flow downhill.
Oddly, if you scoop most of the rivers out it actually starts to work pretty quickly. Are the dynamics on this perfect, hell no, but they at least stand up to a quick skimming over (I could probably get rid of the southern swamp near the Black Knight's Keep-- I'm willing to at least entertain the notion that the Gloomfens are magically created). Credit goes to Havard, whose map I used as a quick'n'dirty template in hexographer when I made this and which you can still see a little of if you look closely.
In the interests of clarity, here's the original drawn map (with, for some reason, the towns removed but a hex overlay added), both so you can see the rivers I removed and in the interests of demonstrating that it seems to me that the general slope of the valley is north to south-- the narrow canyons in the southern mountains are a little weird still, but that seems the implication to me.