These are really cool map ideas, but they're being marred by the lack of detail in geometry and the lack of variety in texturing. You could work on your UV skills as well. The first one, Tributaries, looks like a cool idea, but the way the land looks is really strange. I can say the same about how you have the water flowing. I would revise that a bit, because you've got a river flowing along the top of ridge. Rivers are supposed to take the lower ground. It also looks like part of your river flows uphill (where it's connecting to the large on on the side). Secondly for Tributaries, I'm having trouble seeing balanced gameplay occurring. In multiplayer, higher ground usually means an advantage, because you can hide at the top while being able to cover the whole terrain below. You need to work the landscape to help the lower team get to the top. Don't use too many tunnels; that's the easy way out in my opinion. I would also suggest adding a vehicle path that goes up in switchbacks; keep the vehicle path as difficult or long as by foot.

Your moonbase-like map looks pretty cool; I like the dome and the glass in it. There are only a few things bugging me about this map. What's good is that you've helped balance gameplay by closing off tunnels from the outside. This should interesting for CTF; the enemy getting away with the flag could take any route they want, but it is a long walk to catch 'em if you're defense. You might want to add quick way to get outside from the tunnels. One of the things that bugs me is texturing. The structures almost look too new. Make the textures a bit more grimy, which will create a nice mood for the map. Another thing to help with mood is lighting. The ambient lighting indoors doesn't quite look right. Making it darker and adding some artificial lighting to certain areas would be pretty cool. These are just a few things to keep in mind next time you do radiosity.

All I can say about the last map is possibly some more texture variety. I'm not too sure how gameplay would be on that map. This being said, take a look at Bungie's maps and see how certain terrain and structural elements affect gameplay. Incorporate as needed.

Anyway, great job so far on these! I can't wait to see how they turn out.