This widespread northern species extends south to c. La. along the Mississippi Rv., but its southeastern limits remain somewhat uncertain (Duncan 1967; Moore 1991; W). It is scattered along the Ohio Rv. and Kentucky Rv., but seems virtually absent along the Green Rv. and Cumberland Rv. except near Nashville (Ch). Some colls. are difficult to distinguish from vulpina, and need to be rechecked. Distinctive features of riparia include: relatively large fruits (ca. 7-12 mm versus ca. 5-8 mm in palmata or vulpina), with a heavy glaucous bloom; young leaves enveloping shoot tips, relatively large (ca. 10-20 x 5-15 cm), with lobes sharp and slightly ascending, ciliolate; branchlets terete, dull reddish, with diaphragms only ca. 0.5-2 mm. V. riparia tends to flower earlier than sympatric vulpina, in April-May (versus May-Jun), and hybrids have not been clearly demonstrated (J. Wen, pers. comm.). The species appears to be functionally dioecious in southern Ontario; largely female plants produce inaperturate pollen that does not fertilize ovules (Kevan et al. 1985).