Sewer valves are water-controlling devices found just south of Ardougne. They are used in the Hazeel Cult quest. There are 5 sewer valves total. Each has a "turn-right" and "turn-left" option.
Just east of the Clock Tower is a dungeon entrance. Inside there is a raft. When boarded, this raft goes to different places based on the positions of all 5 sewer valves.
Once boarded, the raft takes you to one of five islands depending on the valves' orientation. The island you land on corresponds with the valve that is incorrect. For example, if the second valve is incorrect, you will land on island two. If the zero valve (the Western-most valve) is incorrect, then the raft will not leave the shore. If all valves are correct, you will land on the fifth island, which is the island leading to the Hazeel Cult's hideout.
Sewer Valve Numbers. When a valve is incorrect, you will land on the corresponding numbered island.