In contrast to many small, mountainous watersheds in temperate coastal regions, where fl uvial discharge and wave energy commonly coincide, deposition and reworking of tropical fl ood sediment can be seasonally decoupled, and this has important implications for coral-reef ecosystems. An understanding of the interaction between tropical fl ood sedimentation and wave climate is essential to identifying and mitigating effects of watershed changes on coral reefs as urbanization and climate change proceed. Sedimentary facies and isotopic properties of sediment in Hanalei Bay, on the island of Kaua'i, Hawai'i, USA, were used to assess deposition and reworking of fl ood deposits from the Hanalei River in a case study demonstrating the potential ecosystem effects of runoff from a steep, tropical watershed.In Hanalei Bay, the youngest and thickest terrigenous sediment was consistently present near the river mouth and in a bathymetric depression that acted as at least a temporary sediment sink. During this 2 yr study, the largest fl ood events occurred in late winter and spring 2006; substantial terrestrial sediment delivered by those fl oods still remained in the bay as of June 2006 because oceanic conditions were not suffi ciently energetic to transport all of the sediment offshore. Additional sediment was deposited in the bay by a summer 2006 fl ood that coincided with seasonal low wave energy. In most years, fl ood sediment accumulating in the bay and on its fringing reefs would be remobilized and advected out of the bay during winter, when the wave climate is energetic. Turbidity and sedimentation on corals resulting from late spring and summer fl oods during low wave energy could have a greater impact on coral-reef ecosystems than fl oods in other seasons, an effect that could be exacerbated if the incidence and sediment load of tropical summer fl oods increase due to urbanization and climate change.
C oral reef ecosystems are threatened by unprecedented watershed changes in the United States and worldwide. These ecosystems sustain fishing and tourism industries essential to the economic survival of many communities. Sediment, nutrients, and pollutants from watersheds are increasingly transported to coastal waters, where these contaminants damage corals. Although pollution from watersheds is one of many factors threatening coral survival, it is one that local people can have a profound influence on. U.S. Geological Survey scientists are using mapping, monitoring, and computer modeling to better forecast the effects of watershed changes on reef health. Working with communities in Hawai'i and on other U.S. islands in the Pacific, they are helping to provide the science needed to make informed decisions on watershed and coral reef management.
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