Is it ensure, insure, or assure?
1849, from river + -ine (1). French form riverain is attested from 1858.
Relating to, formed by, or resembling a river.
Relating to a system of inland wetlands and deep-water habitats associated with nontidal flowing water, characterized by the absence of trees, shrubs, or emergent vegetation. Compare lacustrine, marine, palustrine.