The general physics concept is moment of inertia. If water is moved uphill, this redistributes some of the Earth's mass further away from the axis of rotation, which increases its moment of inertia. Since the total angular momentum of the Earth is not changed by doing this, the result is a reduction of the angular velocity, in other words lengthening the day.

NASA calculated, for instance, that filling the Three Gorges Dam reservoir would lengthen the day by 0.06 microseconds.

I was curious:

https://en.wikipedia.org/wiki/Day_length_fluctuations