The Si rises in the southern foothills of the Mengshan Mountains (蒙山), then flows through Sishui County and the cities of Qufu and Yanzhou before emptying into Lake Nanyang (南阳湖).
Tributaries such as the Fan (反), Sui (睢), Tong (潼) and Yi (沂) swelled its banks as it passed through present-day Yutai, Pei, Xuzhou, Suqian, and Siyang counties in Shandong and Jiangsu.
Its confluence with the Huai occurred at Sikou (泗口) or Qingkou (清口) at present-day Huai'an in Jiangsu.
[2] In 486 BC, King Fuchai of Wu built the Hangou Canal (t 邗溝, s 邗沟, Hángōu), connecting the Huai and Honggou to the Yangtze River to their south.
[3] Amid his ongoing wars against Qi and Jin, in 483 and 482 BC, he further expanded this network with the Heshui Canal (t 荷水運河, s 荷水运河, Héshuǐ Yùnhé), connecting the Si with the Ji River,[3] which ran parallel to the Yellow River through densely peopled districts in what is now western Shandong.