Blackburnia riparia, new species is described from the summit of Mt. Waialeale, Kauai, Hawaii, incorporated into a phylogenetic analysis of Blackburnia Sharp, and thereby placed as the adelphotaxon to two other allopatric Kauai species, B. lata Liebherr and B. atra Liebherr. The new species occurs in riparian habitats, including vertical rock-faced seeps and algal mats, and is documented to feed on aquatic larvae of Micropsectra Kieffer (Diptera: Chironomidae). Blackburnia elegans (Sharp) adults also occur in streams on Waialeale summit where they were found walking under water on the undersides of large, flat rocks. Based on documented prey and observed behavior, both species appear to forage under the waterline. The abdominal and elytral anatomy of adult B. riparia and B. elegans is suitable for retention of a subelytral air bubble, suggesting these species respire underwater. Adults and associated larvae of Blackburnia mandibularis Liebherr inhabit moss-mats along falls' margins at Waipoo Falls, Kokee State Park, Kauai, indicating that this species' active life stages also reside in the riparian zone. Attributes of the larval head capsule and mandibles for B. riparia and B. mandibularis are associated with homologous characters expressed in adult anatomy, ontogenetically linking larval and adult head and mouthpart specializations. The various evolutionary origins of riparian habits are examined across Blackburnia. Via one route, occupation of the montane riparian zone has evolved from terrestrial moss-mat habitation, with concomitant evolutionary reduction of adult compound eyes. A second evolutionary route to the riparian zone is based on occupation of open, disturbed or ephemeral habitats, in some cases assisted by adult winged dispersal.