Other canals are constructed in the north in 113 BC. The purpose was to try to bring the region of the grand loop of the Yellow River into cultivation, and thus to settle this area that had served as an ideal corridor for the invasions of the Xiongnu barbarians.[412] These irrigation canals are built on either side of the Yellow River in the region of the present — day Yinchuan. Their construction had in fact been begun around 215 BC under the reign of the first emperor, and saw further new development under the Ming. In the end, they attained a length of about 180 km.
Between 38 and 34 BC a dam-reservoir called the Maren dam was built near the city of Nanyang on one of the principle north tributaries of the Han river. The project is attributed to Zhao Xincheng, who was prefect of Yanyang. This earthen dam is 16 m high and 820 m long, and includes six gates of stone to control flow into the irrigation canals.[413]
