Older answer but it may help you out. Quite interesting, I thought.
Dams for Water Supply Are Altering Earth's Orbit, Expert Says ...
I need a quick answer for a science question: "How has the water storage affected the rotation of the Earth?"
I heard about this a few months ago. It has more to do with climate and natural water storage rather than artificial water storage.
www.scientificamerican.com/article/climate-change-has-shifte