How would the spout be different from a fortification, and for that matter how would a creek be different from a normal tile?
Uhh... did you read anything I posted?
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 essentially, you get to direct water flow
without digging down to the next Z level or having puddles on your floor. The "spout" would basically just allow these channels to go through walls.
So like this:
~~|[][][][][]~~
~~!----------~~
~~|[][][][][]~~
The ~ on the left is the source water supply, at Z - 1. The [] is normal floor, and the -- are channeled (creek) floor, at Z 0. The | are normal walls, the ! are channeled walls (spout).
The ~ on the right is the destination water, maybe a well or cistern, which will fill up to Z -1, although it will fill slowly.
The z - 1 below the floors are untouched.
You can walk normally on the channeled floors.
Now, for this to make sense, the channel should really only move the water from the left if it is at 6/7 or more. So it's low flow, but allows you to move water around without reserving a Z level for the water to move.
It seems like it'd be too hot to walk over these channels with magma in them though.
It seems quite useful to me. I would use it in my fortresses.