Understanding future sediment transport to coastal waters and coral reef ecosystems in West Maui

Brown coastal waters swirl along a steep cliff coastline.
The health of onshore ecosystems has direct consequences for near shore coral ecosystem health. For example, excessive sediment run-off due to vegetation changes can smother coral reefs.

Coral ecosystems of West Maui support a vibrant tourism industry and provide tangible economic benefits to the community. Hawaiian nearshore reefs generate about $800 million in annual revenue, not including the ecosystem services they provide – such as critical habitat for diverse fish species and buffering coasts from storm surges. The Hawaiian economy depends on healthy coral ecosystems, yet reefs are currently facing multiple threats, including changing climate conditions, local land-based pollution, and sediment erosion.

Erosion of soils into nearshore coastal zones is a chief concern facing land managers in West Maui. Intermittent rainfall can carry sediment from sources such as dirt roads, agricultural fields, streambanks, and disturbed forests into estuaries and the ocean. These sediments can block light that corals need for photosynthesis, essentially smothering them. Sediment also affects coastal user enjoyment by reducing water quality.

The goal of this project was to map, monitor, and analyze recent and historic rainfall to estimate sources of sediment for watersheds in West Maui. Researchers described how much sediment was entering the water from different land-based sources, to create a sediment budget for two watersheds in West Maui. Researchers found that current sediment deposition into coastal waters is likely due to the erosion of streambank terraces located downstream of former agricultural fields. These terraces are remnants of old agricultural land management practices in which soil and rock waste were pushed from fields into streams. It’s estimated that this source results in over 90 dump truck loads of fine sediment entering West Maui’s nearshore environment each year.

The information gathered from this project has already been used to inform a tool for estimating streambank erosion and sediment runoff into coastal zones during rainfall events. This tool can help resource managers and planners assess how different management options could help reduce the amount of sediment entering coastal waters.





Kirsten Oleson
Asst. Professor of Ecological Economics, UH Mānoa


John Stock
Research Geologist, USGS