How do I lower the water even more?

I've already gotten all 64 cube/anti-cube bits in Fez and finished the game, but there are two doors which remain inaccessible to me. The 64-bit door under the little tree island, and the door underneath the main warp gate.

I've already reduced the flow at the waterfall, of course, but I can't find anything else which will allow me to lower the water a little bit more so I can reach these two rooms.

The 64-bit door even opens when I approach, but the water is just high enough that I'm swimming, and entering the door is impossible (i.e. the Up key does nothing).