When reading about Chinese military history, I've noticed that the Yangtze seems to be a major barrier to armies, and that when belligerents on opposite sides of the Yangtze clash, they often fight naval battles to force or prevent a crossing.
The same does not appear to be true of the Yellow and Huai rivers. When someone crosses one of these rivers with an army, it seems like they are rarely opposed by an enemy navy. For example, when the Jurchen Jin dynasty invaded the Song, the Song military was outclassed in all respects by the Jin except for its relatively strong navy. And yet, the Jin seem to have crossed the Yellow and Huai rivers unopposed by the Song navy, capturing the Northern Song capital of Bianjing (modern Kaifeng) before finally being challenged by the Song navy on the Yangtze in 1130. When fighting broke out again in 1161, the Song chose to fight on the Yangtze again rather than defending the border at the Huai.
So what is it about the Yellow and Huai rivers that make them less suitable for naval actions? Are they too narrow? Do they freeze in winter or (partially) dry up in summer?
Turns out they were Portu-Geese
