Accrued water rights

What does Accrued water rights mean in American Law?

The definition of Accrued water rights in the law of the United States, as defined by the lexicographer Arthur Leff in his legal dictionary is:

See accrue. In this context, “accrued” has a meaning particularly close to be vested, for the accrued rights are treated specially in, or even exempted from, whatever statutory provision is passed to affect rights to appropriated waters.