The first and most significant of these is the barrier formed by the Rocky Mountains, which separates the westward flow of water from the eastward flow.
(The first and most significant of these; is; the barrier formed by the Rocky Mountains,) (the barrier; formed; by the Rocky Mountains) (the Rocky Mountains; separates; the westward flow of water from the eastward flow)
The first and most significant of these the barrier the Rocky Mountains
is formed separates
the barrier formed by the Rocky Mountains,
by the Rocky Mountains
the westward flow of water from the eastward flow