Ridge to Reef: West Maui

During storms, sediment-laden plumes turn West Maui’s blue coastal waters to a muddy brown. This sediment pollution degrades the ecological, cultural, and recreational value of these iconic nearshore waters. Using mapping, monitoring, and modeling, the U.S. Geological Survey (USGS) identified and measured sources of fine sediment that pollute the nearshore each year. Although sediment pollution is one of many factors threatening the survival of coral reefs and coastal ecosystems, it is one factor that Maui’s communities can influence profoundly. The USGS findings are being used by local partners to focus their strategies on reducing nearshore pollution. 

Simply put, sediment is dirt in motion. More specifically, sediment is loose particles of rock, minerals, or organic matter that are moving by a process called erosion. The smallest particles - like sands, silts, and clays - can be suspended in water and carried to the ocean. This fine-grained sediment (particularly silts and clays) is the focus here.

Fine-grained sediment can stay suspended in the water for long periods of time. This pollutes clear, blue waters by turning them into turbid, brown water. Suspended-sediment pollution can affect marine ecosystems in many ways. For example, sediment negatively impacts coral ecosystems by: blocking light used for photosynthesis and coral growth, smothering and abrading coral, and triggering increases in macroalgae.

Sediment-laden plumes, or plumes of brown water, are caused by streams flowing below old pineapple and sugar cane fields. Long ago, modification of these fields moved fine sediment into the stream valleys. This legacy sediment is still sitting in the valleys, waiting for streams to carry it the ocean. These streams might only flow a few times per year, usually after hours of heavy rain. When there is flowing water, streams can quickly erode sediment from their banks.

The very biggest storms can erode sediment from other places in the watershed (like old agricultural fields). But these storms are rare, happening only once every few decades. So, frequent brown-water plumes are likely caused by smaller storms eroding streambank material leftover from historical agriculture practices. Buildings and roads can funnel more water to streams, potentially causing worse plumes.

These findings are helping local partners develop ways to reduce land-based sediment pollution. The full USGS report " Sediment budget for watersheds of West Maui, Hawaii ", detailing all the findings, is available for download.

• Fine sediment is already in the streams - stored in streambanks and easy to erode
• These streambanks, only found below old sugar cane and pineapple fields, are the main source of "brown" during yearly storms
• Rainfall - most streams need several hours of heavy rainfall (above 0.2-0.4 in/hr or 10-20 mm/hr) before flowing and causing plumes
• Frequency - heavy rainfall occurs at least 3-5 times per year (maybe more often)
• Erosion rate - streambanks erode during storms at a rate of 0.1-0.2 in/yr (5-24 mm/yr)
• Dump trucks of sediment - almost 100 dump trucks of sediment reach the coast each year (which equals an annual sediment load of 921 metric tons)
• Biggest polluter - watersheds with streams mostly in former agricultural areas are the biggest polluters. For example, Kahana Stream alone likely moves 285 metric tons of sediment a year - equal to 29 dump-truck loads of sediment emptied into the ocean
•  Download the full USGS report and map  - report includes the estimated sediment pollution from each watershed in West Maui and how each was calculated

•  U.S. Army Corps of Engineers (USACE) 
•  USGS National Cooperative Geologic Mapping Program (NCGMP) 
•  West Maui Ridge to Reef (R2R) Initiative 
•  Pu'u Kukui Watershed Preserve 
•  Oleson Lab, University of Hawaii, Mānoa 
•  Coral Reef Alliance 

 Jonathan Stock , USGS project lead and report author

 Corina Cerovski-Darriau , USGS co-author and lead mapper