Again, where would the channel go? How think is a floor?
And what happens to keep a channel from overflowing, and what happens when it does?
The channel goes in the floor. How thick a floor is is a fair question, and really the most pertinent to this discussion. For this to work, a floor must have enough thickness to hold some water without going all the way through to the next Z level. If a tile is 3X3X3 meters, with each tile containing a wall and floor, I suppose a floor could be 1.5 meters deep. So if you cut a 1 meter deep hole into that floor, making it maybe 1/2 meter wide, you could still hold a fair amount of water. So 1/3 as wide as a normal tile, 1/6 as deep, meaning a full channel holds 1/18 the amount of water that a normal 7/7 full tile. Not much water, but still good enough to move water around between normal sized pools.
However, with the way water currently works, a 7/7 full tile with a floor seems to hold the same amount of water as a 7/7 full tile without a floor. This suggests a floor has no thickness whatsoever. But that makes no sense anyway. So I don't care to respect that thickness in my calculations...
Water at the same Z level as the floor wouldn't overflow. If water pressure is too high, due to being fed from greater Z level than the floor, water puddles on the floor (1/7 deep to start), and from there behaves like any other water. With such a limited depth, you'd get a low flow rate, meaning it really wouldn't be able to deal with any pressure without overflowing. However channeled (creek carved by the OP's original description) floors wouldn't have to solely rely on evaporation to drain, so long as the water had somewhere to go.
If you could carve these "creeks" into walls, you could create a flow orifice, meaning a fully flooded room with a "creek" hole leading into an empty room would flood the empty room much more slowly than a full sized carved fortification.
So I say, yeah, that'd be cool to have in the game. But I guess you sort of need to define a floor's thickness for it to make any sense.