(October 2010) The occurrence of the 2008 May 12 Wenchuan earthquake (M 7.9) near the eastern edge of the Tibetan Plateau triggered a debate whether it was influenced by the newly impounded Zipingpu reservoir, located only about 21 km east of the earthquake epicentre. We calculated change in stresses due to the 3-D reservoir water load using Boussinesq theory and pore pressure diffusion due to the actual variation of water level in the reservoir. Using the stresses and pore pressure due to the reservoir impoundment, we calculated the Coulomb stress on the NW dipping fault plane of the 2008 Wenchuan earthquake. We find that the reservoir operation did not cause any increase in the Coulomb stress at the earthquake hypocentre and the region around it, rather it decreased the stress by a nominal amount of about 1 KPa. Thus, we suggest that the reservoir probably did not play a role in the occurrence of this earthquake. However, our analysis suggests that the slightly enhanced low-magnitude shallow seismicity after the reservoir impoundment, but prior to the 2008 Wenchuan earthquake, was positively influenced by the reservoir impoundment.