Overflow Bay, also known as the side weir spillway, is one of the flood control facilities of Jingyuetan Reservoir and is built on one side of the dam. The spillway is used to release flood water when the reservoir capacity of Jingyuetan exceeds the limit, preventing the dam from being destroyed. The spillway includes an inlet channel, a control section, a spillway, and an outlet channel. In summer, when there is abundant rainfall, water from Jingyuetan overflows from the spillway into the Yitong River, Changchun's mother river.