Why did Yongding River become a treasure trove of geomantic omen for royal families in past dynasties?

Yongding River originates from Guancen Mountain in ningwu county, Shanxi Province, flows through Inner Mongolia and Hebei, turns to Hebei via Beijing, joins Haihe River in Tianjin, reaches Tanggu and flows into Bohai Sea.

After the Yongding River moved south, it left a Yuan Ye and a pond with an island named Yaoyu in the water. The terrain is very similar to the pattern of "one pool and three mountains", so it has become a treasure trove of geomantic omen for the royal family in past dynasties.