Water impounded in artificial reservoirs since ~1950 is by far the largest anthropogenic hydrological change in terms of the mass involved. This mass redistribution contributes to geodynamic changes in the Earth's rotation and gravitational field that have been closely monitored by modern space geodetic techniques. We compute the effect of 88 major reservoirs on length-of-day, polar motion, and low-degree gravitational coefficients. On an individual basis much smaller than geophysical signals in scale and magnitude, these anthropogenic effects prove to be non-negligible cumulatively, especially when considering the fact that our results represent underestimates of the reality. In particular, reservoir water has contributed a significant fraction in the total observed polar drift over the last 40 years. ¿ American Geophysical Union 1995