How did the environment affect the diffusion of population in East Asia from Guns, Germs, and Steel?

The answer to this can be found in Chapter 16 of the
book.  There, Diamond discusses why people who spoke Sino-Tibetan languages came to
diffuse themselves all across East Asia until essentially nothing was left of the
original populations in places that originally had ethnically different
peoples.

The reason for this total diffusion was
environmental.  Diamond argues that there were two important environmental factors. 
First, China has long east-west rivers that allow people to diffuse along them.  Second,
there is little in the way of geographical barriers to north-south diffusion.  For these
reasons, people were able to spread in all directions.

East
Asia's relatively open environment with few barriers allowed the population of
Sino-Tibetan speakers to diffuse easily across the region.