I agree it's a strange way of doing it. But, well, that's how it works.
Yes, water works great in DF! I think it's better water in whole game world.
It's probably because a 3/7 stream really isn't supported as well as a 7/7 stream.
A 7/7 stream won't make your computer cry
Sure, I can't talk about game technical reasons, but, 7 means 7 + 0(without water) = 8 bits = 1 byte per tile. So, I dont understand why game haven't different digging status per tile, while it have water(depth) status. Example: digging channels and mines could be implemented like taking water from pit. I.e. dwarf dug 1of7 dirt from tile and put it in dump zone. There we have now small pile of dirt with 1/7 status. So we could not only dig, but fill piles and hills in new place. 4/7 could be slope.
Same status for heavy gases spreading on ground with different levels. I.e. 1/7 poisonous gas don't kill drarf, but mices or cats. I hope your understand what i mean... ))
If you need the river for defensive purposes, just channel it all out. TBH, dry moats and regular walls are much better than water moats, notwithstanding syndrome poisoning.
OTOH, shouldn't major rivers be more than 1 z-level deep? There are a few RL rivers that are hundreds of feet deep in places.
Yes, it is. I can dig channels, but better to have additional water defences like just river at start. Most historical towns were build on river forks to rise natural defence.
I think huge rivers should be deepest.
I do like your idea about making streams not have the water below. We could still have fish in it though and be able to get water from it. If you dug a channel from it, water could flow from it as well, but quite slowly.
Yes, sure. For small stream. Fish could be in ths stream and seen on same level with stream. Maybe, not carps, but small one. Dams will stop flow like in current water model and make it rise. I don't see any contradictions to do that.
It is a strange way of doing it, but it's the best way possible right now without changing fundamental aspects of how water works. If you got rid of the water below the "floor" on top, then you wouldn't be able to get any water out of it at all.
I dont see any needed changing in water behavior. Dig channel near to stream and see how water flows down in it.