Rivera is Spanish for a riverbank or shore—from ribera—used as a habitational name from places called Rivera or as a topographic label for someone living along the water's edge. Catalan and Italian Ribera are close cousins.
It is a place surname, not a patronymic -ez name. US frequency is high among Hispanic families, especially with Caribbean and Mexican migration histories, but the word itself is geography: bank, shore, riverside. Families carried the name across the Atlantic without changing its core sense.