To say nothing of...

The announcement of the shortage isn’t just symbolic. It also triggers mandatory water consumption cuts, which mostly impact Arizona, that take effect early next year. Some 40 million people rely on the river for water, contributing to its decline — to say nothing of the nearly 5.5 million acres of farmland that it irrigates.

Is "the river" an antecedent of the pronoun "it" in the last sentence of the paragraph above?

Is "nothing of the nearly 5.5 million acres of farmland that it irrigates" a NP in which "nothing" is the head of the noun phrase and the PP "of the nearly 5.5 million acres of farmland that it irrigates" its complement?