  Let M be a totally orientable graph manifold with characteristic submanifold
T and let M = V cup_S W be a Heegaard splitting. We prove that S is standard.
In particular, S is the amalgamation of strongly irreducible Heegaard
splittings. The splitting surfaces S_i of these strongly irreducible Heegaard
splittings have the property that for each vertex manifold N of M, S_i cap N is
either horizontal, pseudohorizontal, vertical or pseudovertical.
