Indigenous fishponds are sediment traps for terrigenous sediment on the southern coast of Moloka'i, Hawaii

Terrigenous sediment transport is a major driver of coastal degradation in high-island tropical systems, where steep volcanic watersheds rapidly deliver sediment to the nearshore littoral zone. Indigenous Hawaiian fishponds are engineered coastal embankments with sluice structures that retain land-derived sediments and limit their dispersal across reef platforms. This study quantified the distribution of terrigenous sediment within and adjacent to coastal fishponds along the south-central shore of Moloka'i, Hawaii, using grain-size analysis and elemental composition. We also assessed whether fishpond wall restoration influences sediment retention and whether coastal drainage channels affect spatial sediment patterns across the inner littoral zone. Our results show that fishponds retain significantly higher proportions of clay, fine silt, and land-derived trace elements compared to adjacent open coastal flats, confirming their role as effective sediment traps. While restoration status influences elemental accumulation associated with embankment structure and hydrological connectivity, it does not significantly affect particle-size distribution. Spatial patterns of terrigenous sediment across the inner littoral zone were more strongly associated with proximity to watershed outlets than with fishpond condition. These findings demonstrate that traditional coastal embankments can meaningfully modulate watershed-to-coastal sediment fluxes, highlighting their relevance to contemporary sediment-mitigation and coastal-management strategies in Pacific Island ecosystems. • This study quantifies how traditional Hawaiian fishpond embankments influence the distribution and retention of land-derived (terrigenous) sediments along Moloka'i's southern coast. • We analyzed sediment particle sizes and elemental composition inside and adjacent to fishponds to assess their sediment-trapping function and evaluate restoration effects. • Fishponds retained higher proportions of clay, fine silt, and terrestrial trace metals than open coastal areas, confirming their role as effective sediment sinks. Restoration altered trace metal concentrations but did not significantly change particle-size retention, while sediment patterns outside ponds were driven mainly by proximity to drainage outlets. • These findings highlight the ecological importance of Native Hawaiian fishponds as traditional coastal infrastructure that mitigates land-derived sediment impacts on nearshore reefs and informs contemporary restoration and sediment-management strategies.