It is well known that lithium reacts violently with water under the release of molecular hydrogen and the formation of lithium hydroxide. In this work, the initial mechanisms for the surface reactions of metallic lithium with water from the gas phase were investigated by means of periodic density functional theory calculations. For this purpose, adsorption/absorption structures and diffusion and dissociation processes of hydrogen, OH, and H$_{2}$O on low-index metallic lithium surfaces were investigated. Through thermodynamic and kinetic considerations, negatively charged centers on the surface were identified as the origin of hydrogen formation. The strikingly low reaction barriers for the reaction at these centers implied a self-supporting effect of hydrogen evolution and the associated lithium degradation.