永定河
Lilingyan () was an ancient irrigation system built in 250 AD during the Three Kingdoms Period to irrigate the Beijing Plain around Jicheng (modern-day Beijing). The irrigation system consisted of Lilingyan, a dam across the Shishui (Yongding River) at the foot of Liangshan (Shijingshan), and the Chexiangqu, a diversion channel that carried the water west into the Beijing Plain. The diversion channel fed water to the fields north, east and southeast of Jicheng and emptied into the Gaoliang River, which flowed back into the Yongding River. The irrigation system was an important development in Beijing’s early history and helped increase food production and population in and around the city. Lilingyan was named after "Liling", the tomb of Liu Dan, Prince of Yan, who was buried in Liangshan after his death in 80 BC.