Every day, the calm clear waters along the south coast of the Hawaiian Island of Moloka'i undergo a dramatic change.
As waves caused by the daily trade winds build and the tide rises, muddy sediment on the reef flat becomes resuspended, creating a belt of red-brown water and reducing visibility to a few inches.
The muddiest water is nearest the shoreline and gradually dissipates farther offshore toward the reef crest.
During calmer overnight conditions, the suspended sediment settles back onto the reef flat until the cycle is repeated the following day.
In an effort to quantify the amount of sediment involved in this daily resuspension, scientists from the U.S. Geological Survey (USGS) conducted an experiment in July, using a new digital airborne imaging system and concurrent water sampling.
This experiment was part of a large-scale USGS effort to better understand the effect of geologic processes on coral-reef systems in the United States and its trust territories, an effort that includes ongoing coral-reef studies in Moloka'i (for example, see articles in May 1999, March 2000, and April 2002 Sound Waves) and a recent coastal-circulation experiment off West Maui (see articles in August 2003 Sound Waves and this issue).
The digital airborne imaging system used in July's experiment includes a Duncan Tech MS3100 camera system mounted on the bottom of a helicopter.
The imaging system was designed and assembled by Pat Chavez, Dave Tucker, and Rian Bogle (USGS, Flagstaff, AZ).
The system has three spectral bands (10 bits per band), with one band selected to optimize clear-water penetration. It also has individual band gain control, which is quite useful when imaging low-radiance features, such as water.
Images were collected three times throughout the day along the south coast of Moloka'i: early morning (calm), late morning (medium resuspension), and afternoon (highest resuspension). Between these flights, images were also collected over vegetated sites on land for use in soil-erosion studies.
During the coastal-water overflights, ground-based teams collected water samples with Niskin bottles from both clear and muddy water.
The ground-based teams led by Susan Cochran-Marquez included Greg Piniak and Eric Thompson (USGS, Santa Cruz, CA), Becky Stamski (University of California, Santa Cruz), Henry Wolter and Connie Hoong (USGS Water Resources Discipline [WRD], Honolulu, HI), Eric Wong (USGS WRD, Maui, HI), and Bryan Bailey (USGS Earth Resources Observation Systems [EROS] Data Center, Sioux Falls, SD).
Extra assistance was provided by our friends Tina Lau, Scott Atkinson, Kathy Tachibana, and Niles "Kawehi" Soares from the Nature Conservancy of Moloka'i.
The water samples were analyzed for suspended-sediment concentration, which will be used to calibrate the digital images. This information will be used to calculate the total amount of resuspended sediment on the reef flat during the three overflights.
The new digital airborne imaging system has also been used to quantify sediment in the Colorado River through the Grand Canyon, to help map the benthic habitat of the Colorado River, and to image burned areas from wildfires in the Southwestern United States. It will also be used in September on east Maui to image streams to help with fish-habitat mapping, and next spring to image and monitor dust storms in the Mojave Desert.
in this issue:
Suspended Sediment in Hawaiian Waters