Making pools of water in exterior cells

I am making a house on top of a hill in the wilderness East of Niben Bay ([33, -3] exactly), and I want to make a pool of water on the hill that will flow down into the nearby lake. I've seen waterfalls in the construction set and I know they're objects, but how do I make the pool of water? The cell already has a default water level below the terrain, and I can see the lake has a water level above the rest of the cell (picture). I would also like to know if whatever you do to make two levels of water would work in interiors as well.

I browsed the wiki extensively to try and find the answer, but no luck.--Trigonous 21:55, 28 November 2007 (EST)