Not at all. There have long been obvious differences between north and south china, but the Yangtze isn't the border between North and South, it's the heart of the South. North China is divided from South China by the Huai river (http://en.wikipedia.org/wiki/Huai_He).
Chinese civilization is essentially the story of intensely farming a flood plain. But China has two such plains: the Yellow River defines the north, and the Yangtze defines the south.
