永定河
Lilingyan was built in 250 AD by Liu Jing, a military commander of Youzhou, a prefecture in north China based in Jicheng. Liu Jing was the son of Liu Fu, who had built irrigation systems along the Huai River. As the commander of the local garrison, Liu Jing had to purchase grain from afar due to limited food production locally. To improve local agricultural output, Liu Jing diverted the waters of the Yongding River, which flowed south of Jicheng to irrigate the plains to the north of the Yongding River. He deployed soldiers to dam the Yongding in the hills west of Ji and channeled the water eastward along the Chexiangqu into the Beijing Plain. The dam was 2.4 meters high and piled from woven baskets of stone. To withstand the flash flood of theYongding as the river flows out of the Western Hills, the dam was 72 meters thick and gently sloped so that flood water can flow over the dam. The Chexiangqu made use of the old river bed of the Yongding, which used to flow north of Jicheng before it changed course and flowed south of the city.