Yep so I missed my own challenge. Shame on me.

I voted for Oh Tarn, because I thought it was not only nicely executed, it also had all the elements there, made sense and could even be educational.

But the others were good, too, and I think this is evident by the close race we're having. Feedback:

The Amazonas: I like the map, but in the end it's an existing river and we all know that real rivers follow real physics anyway; the idea of the challenge was to correctly map a river based on the geography around it... and if you don't make it up, then that part of it just doesn't get done, it's predefined, and the challenge is somewhat short-circuited.

Mangrove River Postcard: I love this map, and I can totally imagine this place. It'd make for a great setting. However, the surrounding terrain isn't really defined and as such it's hard to say whether or not the river system works. There are no mistakes in it ... that I can tell (I am not a specialist, by any means). If this had had something to indicate where the watersheds/drainage basins are... anything to show why the rivers are the way they are, it would have been my clear favorite.

It's A River: A beautiful map, I like the style a lot. I considered to vote for thisthis, but in the end I think there are actually two problems with the river. Not major ones, mind you; one, I am not sure why the lake outflow is in the NW - next to a mountain. Should this terrain not be elevated compared to, say, the SW side of the lake? The other problem I see is just west of the black city, where the river seems to run straight through a small mountain range. Not impossible, and I am probably nitpicking, but in the end this cost It's A River my vote.

I am still very impressed with all of these maps, can't say that enough.