Murky Waters


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Murky Waters

USGS tracks sediment on Molokai’s reef

By Catherine Cluett (Molokai Dispatch)
July 2010

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Above: One of the sediment collection pods being recovered from 55 ft of water off Kamiloloa during a USGS study of turbidity on Molokai's reef. Photograph by Josh Logan, USGS. [larger version]

[Reprinted, with permission, from the Wednesday, June 9, 2010, issue of Molokai Dispatch]

Ever looked at the muddy water off Molokai's south shore and wondered what it would take to clean up the reef? That's the question researchers from the United States Geological Survey (USGS) looked to answer last month with a study on the reef's turbidity, or murkiness.

The study is part of a larger ongoing study that examines the effects and possible solutions to erosion mauka-side [on the mountainsides] that results in sedimentation of Molokai's reefs.

"Anyone who lives on Molokai knows how brown the water gets," said lead researcher Mike Field, a marine geologist with USGS. In 2008, Field co-authored The Coral Reef of South Molokai, Hawaii: Portrait of a Sediment-Threatened Fringing Reef, a nearly 200-page full-color report [http://pubs.usgs.gov/sir/2007/5101/].

Field questioned how long it would take for natural processes to clean up the reef if erosion from the uplands ceased. To find out, he looked at sediment particle concentration and the rate at which those particles travel. Field and his team completed the research in four days in May.

"We're still trying to learn things that we had half answers for," Field said.

Based on initial estimates, he said it will take sometime between 10 and 30 years for natural processes to clean up the reef if contributing erosion is halted.

The Process

Turbidity is a natural process caused by waves and ocean currents. Waves redistribute sediment and transport it across reefs. On a calm day, the reef looks relatively clean, Field said, whereas trade winds make the water look murky.

Field and his team picked conditions and time of day when turbidity would be at its peak: the highest tides of the month and in the afternoon when the tide is turning and trade winds are blowing. Field placed instruments on the reef floor to measure currents, water temperature and collect sediment samples. They also ran a picket line between Kawela and Kaunakakai Wharf to measure where turbid water flushes out of the reef as the tide turns to calculate how much sediment is being washed out and how fast it is moving.

"We [now] know how many particles leave the reef every day," Field said.

In addition, Field took advantage of another tool to study water flow mauka to makai [from mountains to the sea]—black carbon particles from last year's brushfire that burned 8,000 acres of Molokai's hillsides. Collecting the tiny pieces of carbon that had washed into the reef proved an indicator of direct run-off and how far the particles traveled since the fire.

Field and his team are now analyzing their results. He said the study will probably be published in a journal by the end of the year.

"We've done computer modeling and we're starting to get a good idea," Field said.

The Variables

Those natural processes, however, are dependant on many variables—most importantly, what happens in mountain regions overlooking the reefs.

That's where collaboration with other scientists studying vegetation and erosion comes in. Jim Jacobi, another USGS scientist, is simultaneously compiling vegetation maps of the watershed area, plant distribution, and trends in ungulate (goat and deer) populations that directly affect erosion rates.

Jacobi found that when the Kawela watershed was first sampled in 2008, over 99 percent of the area was bare ground, largely due to grazing feral goats. When the same area was surveyed again last year, plant cover had increased by 27 percent.

Another USGS colleague, John Stock, found that the rate of erosion on the Kawela mountainside today is about 100 times higher than the rate at which an island would normally erode.

"If the hill slopes were re-vegetated [it] could vastly slow erosion to what it should be," Field said. And that would reduce the turbidity we see on the Molokai reef, he added.

Other variables in the equation include rising sea levels, which will change the energy and dynamics on the reef and lead to possible increased shoreline erosion, according to Field.

"Molokai is an ideal place to work," said Field. "[Turbidity on the reef] is a very real problem." He added he has also received great cooperation from residents and assisting organizations such as The Nature Conservancy.

Future turbidity studies may be even more high-tech. Field mentioned a "tracer project" he's planning for next year that will trace individual particles from the Kawela watershed to the reef.

Reversing Coral Reef Decline in Hawai‘i—a New Look at a Critical Problem
January 2009

Coral-Reef Investigation Featured in the Molokai Times
January 2008