One interesting aspect of the structure of Fallingwater which I’m seeing much more clearly after building this model is how each level of the building has the logical structure of a “pool.” The floor layouts are open and in slightly irregular shapes, with edges and patterns which mimic the flow of water; the connections between levels, both inside the building and outside (through the various paths and balconies) indicate the natural way in which water would fall from one level to another in a natural system of pools on a cliffside. The windows serve to both open up the space and let in light, and to highlight this building / water relationship. (The model designer nicely used the same brick type for the water and the windows, which really makes this vivid) The river at the bottom simply seems to be the lowest level of this stacking. The overall effect is that the house is like a permanent (but oddly well-sheltered) camp in the hills.
I think I need to get out to Mill Run sometime and see this in person.