In part four of Walking the Beijing Waterways, D J Clark starts where the last walk finished, at the entrance to Yuyuantan Park under the old CCTV Tower in the far west of the city. It's the longest of the five walks at 26 km and takes a full day to complete. Divided into two sections, the first follows the Yongding River to where it meets the city moats of the Qing Dynasty and follows them in a horse shoe around to the Beijing Railway Station where it meets the Tonghui canal. The second section follows the water east towards the Grand Canal.