The sediment budget of Hanalei Bay, Kauai, Hawaii

The sediments of Hanalei Bay are separated into two groups based on their source: terrigenous and marine. The terrigenous component, primarily from the Hanalei River, was analyzed with a combination of field measurements and models. Field measurements included suspended sediment concentration in the river as well as thickness and calibrated radiocarbon ages of the cored fluvial deposits on the coastal plain. An empirical model, the Universal Soil Loss Equation (USLE), was used to describe the erosional characteristics of the Hanalei basin and to estimate the amount of sediment lost from the valley due to water erosion. The marine component was investigated by combining observational data with geophysical surveys. Benthic samples, collected from numerous sites throughout the bay, show the physical characteristics of the sediments, while side-scan sonar and seismic reflection surveys disclose the lateral extent and sediment thickness.;Suspended sediment and floodplain volumes indicate 7,560+/-2,910 Mg (metric tons) yr--1 are removed from the Hanalei watershed. This is a much higher percentage (45--101%) of the USLE-predicted hillslope output (4,800+/-5,600 Mg yr--1) than would be expected from a watershed the size of Hanalei (54.4 km2). The excess output of fluvial sediment is likely due to other erosional processes such as mass movement and channel incision.;Calcium carbonate sediments, which comprise approximately 70% of marine sediments in Hanalei Bay, are composed of the skeletal remains of marine organisms such as coralline algae, coral, molluscs, foraminifera, Halimeda, bryozoa, and echinoderms. Detrital sediment grains are the most common individual grain type (∼27%). The bay holds approximately 45.5+/-1.5 x 10 6 m3 of marine sediment while an additional 33.7+/-11.2 x 106 m3 underlies the modern coastal plain. Deposition of this sediment likely started soon after the marine environment entered the bay ∼11.7 kyr. Based on CaCO3 production rates in the literature, this is more carbonate sediment than the reefs in Hanalei Bay could have produced in the given time. As a result, an average of 2,490 m3 yr--1 were calculated to have been transported and deposited in Hanalei Bay by the northeast tradewind driven longshore current.