Tom Elwood -best of the internet.: Three Gorges dam slowed the earth's spinning

Picture a figure skater spinning with arms stretched out. When the skater pulls in their arms, the spin speeds up; when they extend their arms, they slow down. Similarly, Earth's spin slows by a tiny amount when more mass moves away from its axis. The reservoir is like the skater's arms stretched out, causing a subtle slowdown of about 0.06 microseconds in the length of each day.