Soil redistribution in the Hanalei River watershed, Kaua’i, Hawaii

The Hanalei River delivers suspended sediments and organic matter to Hanalei Bay which has important implications for the sustainability of coral reefs and their many associated species in the Bay. The identification of sediment source areas in the watershed is a key component for consideration in the design of management strategies to reduce sediment and chemical loads to the Bay.  The objective of this study is to determine the sediment sources within a watershed, such as sheet and rill erosion (upland soils), stream bank and channel processes, and mass wasting.  In order to understand the relative contribution from each of these sources, soil samples were collected from the various geomorphic surfaces within the Hanalei River Watershed and analyzed for fallout Cs-137 concentration.  Fallout Cs-137 can be used as a tracer or fingerprint to identify sediment sources and as a marker to determine floodplain deposition patterns and rates within the watershed.  For this study, recently deposited sediments on floodplains and the Hanalei bay were compared to upland sediment sources (upland soils and mass wasting sites) and stream bank samples using a simple mixing model to determine sediment sources. Cs-137 concentration varied with upland soils > colluvial slopes > floodplain deposits > stream banks > Bay sediments. Preliminary results indicate that channel banks and mass wasting are probably the most significant sources of sediments deposited on the floodplains and in the Bay.