100 Years of Legacy Sediments: Forest to Fields & Roads

Stabilizing sediment and restoring native vegetation is critical to restore the entire watershed, from the landscape to the coral reefs.

Historic land use changes over the last one hundred years have changed the landscape from a native plant dominated forest into terraced field blocks linked by a network of dirt agricultural roads.

These fields were used for sugarcane and pineapple cultivation as well as cattle ranching in the upper edges of the agricultural district. Although these activities have now ceased, fallow fields and abandoned roadways continue to contribute increased sediment and stormwater runoff flows to nearby streams. The landscape’s ability to store and infiltrate stormwater is extremely impaired.

When storms move across the landscape, flowing rainwater picks up sediment and other pollutants, carries them into streams, and out into the ocean.

West Maui has significant legacy sediment deposits which were pushed into stream gulches from fields above. According to USGS, these legacy deposits represent an estimated 40-70% of the sediment load in this area and are a significant source of sediment to downstream reefs.

Preliminary findings suggest stream sediments are mobilized even by relatively small rain events. Large storm events moving huge volumes of sediments off of roads, scouring through upland areas that were intensively farmed for the past 100 years and the associated dirt road network, and carrying the sediment out to the ocean.

Restoration of the landscape with native vegetation and reduced erosion can jump-start healing of the watershed from the highest ridges to the deepest reefs. A watershed in balance maintains its own balance; targeted restoration can efficiently guide the watershed back to health and resilience.