The authors report accurate quantum dynamics calculations for the title reaction on the three lowest electronic state potentials. The adiabatic pathway on the ground electronic state (1(1)A') of H2O has a complex-forming mechanism, manifested by rotationally hot and vibrationally cold OH products with a nearly forward-backward symmetric angular distribution. As energy increases, the adiabatic pathway via the 1(1)A '' state and nonadiabatic pathway via the 2(1)A' state become significant. The former has an abstraction mechanism and produces an exclusively backward differential cross section. On the other hand, the latter has essentially the same dynamic signatures of the ground-state pathway. The inclusion of the two excited-state pathways is necessary to quantitatively reproduce the observed rise in the integral cross section at high energies and the increasingly backward bias in the differential cross section. It is also found that the inclusion of the excited-state dynamics, particularly the nonadiabatic 2(1)A' pathway, greatly improves the agreement with the measured rate constant.