If you want to run this experiment yourself (it should be easier to examine it closely than by watching a movie), I've uploaded the save folder (7zipped) from just before I started the experiment. You should be able to tell what levers control what in the experiment by looking at what they do in the movie, mostly. (There are a lot of other devices and levers scattered around the map, though)
Save (no mods are installed): http://shadowlord13.googlepages.com/grandWaterPressureExperiment.7z
With this complex setup I performed several experiments, which you can see occuring in the movie. I tested the following things:
1. Does water which comes out of a pump have 0 pressure? Answer: Yes! (When the water fills the 3x3 room, if the answer was no, it would have flowed up into the 1x1 space above the center of the room)
2. Can water flow upward and to the side at the same time? Answer: No! (If the answer was yes, it would have flowed from the pump's walkable tile up through the gear channel which was above the unwalkable tile)
3. Does water which falls and becomes spread out to less than 7/7 retain its pressure and transmit it to any other water it touches? Answer: No! (If the answer were yes, when the two groups of water were combined, and the pump to the right was still on, the water would have flowed up the open shaft on the left, and would have flowed into the 1x1 tile above the center of the 3x3 room.)
Note on question 3: There are several possible reasons this could have gone the way it did (further experiment may be necessary (if possible) to determine which is the cause of the observed behavior):
Either falling water does not retain pressure, or water which drops below 7/7 loses whatever pressure it had, or pressure in a 7/7 water tile is determined by the highest 7/7 water tile the current 7/7 water tile is connected to.
(Other experiments have provided evidence that high-pressure water does transmit its pressure to low-pressure water, but that pressure is only transmitted by 7/7 water)
The next thing to test would be whether pressure is transmitted instantly, or whether it takes time. How? By refilling the river (I dammed it), and then putting a pump on one end of it which pumps water up a level into an enclosed 1x1 space. If every tile of the river overflows simultaneously, pressure is transmitted instantly. If not, it probably takes time to be transmitted.