Due to the low water temperature in the Kosai River, the main water source for the Takinoyu Weir, Lake Tateshina was constructed as a reservoir to warm the water.
Since the location of the reservoir was the water source for the Kubota Weir, there was a need to divide water between the Takinoyu Weir and the Kubota Weir in a 9:1 ratio in keeping with an arrangement from the Edo Period, so a “circular tank diversion works” was constructed. The water spilling from the cylindrical tank is accurately divided by the ratio of the circle’s circumference, and this decreased fighting over water.