Every bare dirt pile on Maui is one rainstorm away from the ocean. Sediment is one of the biggest land-based threats to Maui's reefs, and excavation sites are part of the story.
What sediment does to coral
When sediment covers coral, it cuts the coral's ability to photosynthesize and grow and lets algae take over. That's how University of Hawaiʻi researchers describe it. Between 2000 and 2015, coral cover on West Maui's northern reefs fell from about 30 percent to 10 percent, likely driven by sediment runoff during rain events.
Where the dirt comes from
U.S. Geological Survey research in West Maui found that much of today's sediment comes from eroding streambank terraces downstream of former farm fields, an estimated 90-plus dump truck loads of fine sediment a year. NOAA reported that debris piles from past land clearing were a key source. Unpaved roads are another. Today's construction adds to that load if sites aren't controlled.
A 2025 Nature Conservancy study of Olowalu called its 939-acre reef a critical source of coral larvae for reefs throughout Maui Nui. Protecting it means controlling sediment upslope.
What good erosion control looks like
- Phase the work: don't strip the whole lot at once.
- Perimeter controls: silt fence or fiber rolls on the downhill side.
- A stabilized entrance so trucks don't track mud onto the road.
- Covered stockpiles: tarp or mulch dirt piles before rain.
- Protected drains and gutters so sediment doesn't enter the storm system.
- Fast stabilization: mulch, hydroseed or plant as soon as grading is done. West Maui's Ridge to Reef effort has used vetiver grass plantings.
It's also the law
Hawaiʻi's soil erosion and sediment control law (HRS Chapter 180C) underpins the county grading program. Maui County grading applications ask for erosion control measures and best management practices, and the local Soil and Water Conservation District reviews grading plans. See the permit guide.
Related: drainage for Kona storms and driveways that don't wash out.
Sources
- UCSB / UH: Cost-effective solutions to sediment runoff in West Maui
- USGS: Sediment transport to coastal waters and coral reefs in West Maui
- NOAA Digital Coast: Identifying sediment sources in Maui
- The Nature Conservancy: Olowalu reef and land-based sediments (2025)
- Maui News: Upstream solution is key to saving reef (vetiver plantings)
- UH Sea Grant: HRS 180C and county grading permits