An aerial view of Molokai. Note the thick sediment wash on its south shore. Photo courtesy of NASA

The Hawaiian island of Molokai is making a mess of its south shore reef. Every time it rains, gray ashy soil from the ancient volcanoes that formed the island wash downhill to the shore below and pollute the reef.

It wasn’t always this way. Before humans first came to the island, the mountains were covered with plants and shrubs that kept erosion in check. The plants anchored the soil and kept it from washing away when the heavy fall rains began. But now, wild goats introduced to the island by early settlers around 1500 have multiplied are stripping away the plants that anchor the soil. Some soil has always washed down the mountain and into the ocean with seasonal rains, but now, with more goats eating more vegetation, the problem has become more serious. Stephen DeLong of the University of Arizona says that some of the areas cleared by the goats are washing 100 times more sediment into the ocean than they used to.

Standing on the south shore of Molokai and looking up towards the mountains above, the problem is obvious. You can see the washed out areas clearly. And when it rains, brown silty rivers about 3 centimeters deep form and rush down the mountains.

DeLong estimates that these rivers deliver about 6,000 metric tons of sludge each year to the near shore waters and the fragile coral reef below. He is working with the US Geological Survey to figure out the details of how the sediment finds its way from the ridge tops to the reef. He has deployed an arsenal of specialized instruments along the wash patterns to see which spots in particular are producing most of the sediment, because not all bald spots generate the same amount of sludge. The idea is to pinpoint the “hot spots” of sludge production so that they can come up with a plan to mitigate the problem and save the reef. In his talk yesterday, Sediment budget for a polluted Hawaiian reef using hillslope monitoring and process mapping EP22A-01, DeLong said that if global warming intensifies storm events and accelerates loss of vegetation in the area, the problem could get a lot worse.

–Donna Hesterman is a graduate student in the science writing program at UC Santa Cruz