Tropical islands support unique communities of endemic amphidromous species, with life cycles necessitating connectivity between freshwater habitats and the ocean. Simultaneously, surface water diverted at low-head dams is a critical source of municipal and agricultural water supply in resource limited islands. Water withdrawals at low-head dams can dewater downstream habitat and limit connectivity to upstream habitat, with negative consequences for the recruitment and survival of amphidromous species. After approximately seven years of full flow restoration (2016-2023), water withdrawals restarted in some East Maui streams, providing a unique opportunity to study the consequences of partial flow restoration. We conducted ecological surveys at two elevations: in 2025 at lower elevation reaches to test the hypothesis that there is no statistical difference in habitat availability, the density and abundance of species, or community composition of aquatic macrofauna (e.g., fish, crustaceans, mollusks) among streams with partial flow restoration compared to streams with full flow restoration; and in 2024 at upper elevation reaches to test the hypothesis that recruitment of an amphidromous mountain shrimp, Atyoida bisulcata, is not affected by low-head dams with partial flow restoration compared to full flow restoration.
Despite differences in streamflow characteristics, in lower elevation reaches we found no significant differences in habitat availability, the density or abundance of species, or community composition among streams differing in flow restoration. We observed greater densities of the transient freshwater Kuhlia species in streams with open estuaries compared to estuaries closed by wave action, independent of streamflow. At upper elevation reaches, populations of A. bisulcata were not significantly different in streams with partial flow restoration compared to full flow restoration. We conclude that partial flow restoration, in combination with frequent, naturally occurring runoff events that overtop low-head dams, drive recruitment and reproduction, substrate reorganization, and habitat heterogeneity, all supporting endemic freshwater species.