How does the Cistern work?

[Jack]

I turned first valve, then second valve. I left area. After some time I came back and turned second valve again. Then organic stuff spawned (not lurker), and I left the area. Then I continued and I finished whole game without passing the bridge (the note "Bridge must be lowered to proceed" stayed in mementos till the end.) Also I did not have to do some puzzless like putting pipes.