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USGS study finds soil deposits help cause brown water events — even during small rainstorms

By Staff | Oct 18, 2018

A CORAL volunteer installs native plants in soil accumulated in front of a vetiver “eyebrow” as part of a road decommissioning project in Wahikuli Watershed designed to reduce erosion. PHOTO BY TOVA CALLENDER.

WEST MAUI – Brown coastal waters are an all too familiar sight in West Maui.

They interfere with our enjoyment and use of the ocean and stress out the various forms of life that call the nearshore waters home. They can and do cause illness in humans.

Much energy at many different levels is being spent to address this problem. The underlying issues that bring brown – or “turbid” – waters with each rainfall can seem like too much to tackle.

However, the many agency and community partners of the West Maui Ridge to Reef Initiative (R2R) decided inaction is not an option. They have been steadily planning, studying and piloting projects for the last several years intent on making a difference.

The first step in getting to a solution is understanding the problem. In the case of rainfall-generated brown water events, this means figuring out where the sediment is coming from.

At first pass, it seemed the unmaintained network of agricultural roads up slope were the main culprit, but after a few years of study by Geomorphologist John Stock from the U.S. Geological Survey, another source emerged.

His study shows that a century of plantation agriculture requiring extensive land and water-flow manipulation on the slopes of Pu’u Kukui resulted in accumulations of soil in the gulch bed and banks.

This means farming accidentally deposited a lot of topsoil in the streams and gulches below. These sediments move readily with even small amounts of rain. While natural erosion has shaped our gulches over geologic time, the rates these legacy agricultural sediments are moving out to the ocean is roughly ten times the background level.

The agricultural roads and fields are not, however, off the hook; the Stock study found that if more intense rain falls – such as in a ten-year storm – they also contribute to the problem at the coast.

Partners in the R2R have been working on ways to stabilize soil in grant-funded efforts, mostly in Wahikuli and Honokowai watersheds (above Kaanapali), but since the sources of sediment vary, so must the interventions.

To address sediment sources in the former fields, non-profits, the Coral Reef Alliance (CORAL) and Ridge to Reefs have been planting vetiver, a deep-rooted, sterile, clumping plant to create soil catchment areas in the channels (called “kick-outs”) spaced at intervals along farm roads. While designed to remove water from the roads, these kick-outs also accelerate the delivery of soil into the gulch and eventually ocean, but with eyebrows of vetiver forcing the sediment to settle out, more soil is staying put on the landscape.

CORAL and R2R have also been decommissioning unused roads, stabilizing areas with vegetation and advocating for vegetated gulch setbacks in the community plans and county code to reduce the impacts of the next generation of land alterations.

Reducing what is already settled into the gulch bed and banks (sediment mobilized in small rain events) requires different tactics. The known approach is to work with the series of sediment basins located in many of the West Maui streams just above the highway and out of sight. Many of these structures were installed with the help of the large landowners and the Soil and Water Conservation District in the 1980s and ’90s, and while they led to large improvements in coastal water quality, modifications are needed to further increase the amount of soil they catch when it rains.

This is critical, since water quality testing has shown we are almost always in violation of state water quality standards for turbidity, and we know our coral reefs are severely impacted.

For an update on coral status in West Maui from the Division of Aquatic Resources, come to Science Night at Kohola Brewery on Oct. 24 at 6:30 p.m.

The West Maui Soil and Water Conservation District is leading a project at the Kahana Nui Basin to restore storage capacity and help the Department of Public Works access the broken valve that has prevented regular maintenance. If all goes well, by the next hurricane season, the waters in Kahana will not be as severely impacted by turbid water as they were this season.

The U.S. Army Corps of Engineers is also working on new ideas to deal with this challenging source of sediment, the first pass of which were presented in a public meeting in August.

The R2R is far from claiming success at addressing the brown water issue, but as the steady momentum builds, so does the science to back the need for investment in solutions, and pilot-scale efforts create the way for more impactful interventions.

The sources of sediment driving coastal brown water is one of many stressors impacting coral reef health that R2R partners are working on, along with nutrient inputs, stormwater interventions, wildfire fire preparedness, ecological and physical monitoring, and addressing gaps in best management practices that could be employed by property managers, contractors and more. Grit and endurance are necessary for this endeavor, along with a diversity of skill sets, perspectives and the commitment to stay hopeful.

Being a ridge to reef undertaking, action on land is with an eye towards the ocean, with a great example of early promise in addressing one coral stressor in the herbivore management approach in North Kaanapali. The Kahekili Herbivore Fisheries Management Area protects the “lawn mowers of the reef,” helping to keep the growth of algae down so that coral have one less battle to fight.

In celebration of this unique area, the Ridge to Reef Rendezvous will be held at Old Airport Beach (Kahekili Beach Park) on Saturday, Oct. 27. This event is free and open to the public, with a Keiki & ‘Ohana Fishing Tournament starting at 8 a.m., and the general event running from 9 a.m. to noon.

Many of the engaged and passionate people making a difference in West Maui will be in attendance, hoping to meet new members of the community who want to better understand the issues or get involved. If further enticement is needed, come for the games, haunted reef tour, amazing prizes and delicious food.

Kahekili Beach Park is privately owned and maintained by the Kaanapali North Beach Masters Association at their sole expense. This beach park property and its facilities are made available to the public for primarily casual park uses and beach access; subject to signage at the park which clearly indicates and states rules and regulations for use and enjoyment of the facilities during posted hours only and strictly prohibiting commercial uses.

For more information on the event and all R2R projects underway, visit www.westmauiR2R or e-mail WestMauiKumuwai@gmail.com.

Tova Callender is the West Maui Watershed & Coastal Management coordinator for the West Maui Ridge to Reef Initiative.