The answer to this can be found in Chapter 16. Specifically, it can be found on p. 331 in the paperback edition of the book. The answer, not surprisingly, is that rivers run from the inland to the coast and thereby facilitated the diffusion of crops and technology.
China has two large rivers that run from the inland to the coast. In both cases, the rivers run through gentle terrain for much of their length and are therefore navigable. These rivers are the Yellow River in the north and the Yangtze River in the south.
Diamond emphasizes this point because one of his major themes in this chapter is that China was easily unified. He wants to emphasize that crops and technology could diffuse from place to place and that governments could easily control a wide area of land. This led to a situation in which China was politically unified for much of its history.
So, the answer here is that rivers were important in allowing diffusion in China. As Diamond says,
…China’s long east-west rivers … facilitated diffusion of crops and technology between the coast and the inland.