What is a watershed moment?
A critical turning point.
What it is actually is a moment in time where everything
changes. A point in time when nothing after will ever be the same
as before. To call it a turning point technically is true, but it
is an overly simplistic definition of the phrase.
The figurative meaning comes from the literal meaning of a
point, or division in a river, or stream where the river is split
into two distinct paths that will not intersect again