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United States Department of Agriculture

Agricultural Research Service

Research Project: QUANTIFYING LANDSCAPE FACTORS INFLUENCING SOIL PRODUCTIVITY AND THE ENVIRONMENT Title: Determining sediment sources in the Hanalei River Watershed, Kaua'i, Hawaii

Authors
item Ritchie, Jerry
item Pedone, Paul - USDA, NRCS, PORTLAND, OR

Submitted to: International Soil Science Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: May 1, 2008
Publication Date: May 19, 2008
Citation: Ritchie, J.C., Pedone, P. 2008. Determining sediment sources in the Hanalei River Watershed. In: Proceedings of the International Soil Science Conservation Organization Conference (ISCO2008) Congress, May 18-23, 2008, Budapest, Hungary. 2008 CDROM.

Interpretive Summary: The Hanalei River in Kaua’i, Hawaii delivers suspended sediments and organic matter to Hanalei Bay limiting the growth and sustainability of coral reefs and their associated species in the Bay. Concern about the Bay’s health has lead to studies to understand the sources of sediments from the watershed so that management strategies for reducing suspended sediment loads to the Bay can be developed. This study used Cesium-137 to determine potential sources of sediments from the watershed. To understand the relative contribution of the different geomorphic surfaces (i.e., upland soils, colluvial deposits, floodplains, stream bank, channels) samples were collected from these various geomorphic surfaces and analyzed for Cesium-137. A simple mixing model was then used to determine which geomorphic was contributing the greatest sediment to the stream. 137Cesium concentrations in the different geomorphic sources varied with upland soils > colluvial slopes > floodplain deposits > stream banks > Bay sediments. These results indicate that stream banks are probably the most significant sources of sediments deposited on the floodplains and in the Bay and should be targeted for the greatest management efforts.

Technical Abstract: The Hanalei River in Kaua’i, Hawaii delivers suspended sediments and organic matter to Hanalei Bay with impacts on the sustainability of coral reefs and their associated species in the Bay. Understanding the sources of sediments from the watershed is necessary for the development of management strategies for reducing suspended sediment loads to the Bay. The objective of this study was to determine the sources of sediments within a watershed, such as upland soils, stream bank, channels, and mass wasting. In order to understand the relative contribution from each of these sources, soil samples were collected from these various geomorphic surfaces within the Hanalei River Watershed and analyzed for Cesium-137. Fallout Cesium-137 can be used as a tracer or fingerprint to identify potential sediment sources and as a marker to determine floodplain deposition patterns and rates within a watershed. For this study, recently deposited sediments on floodplains and the Hanalei bay, and stream samples were compared to upland sediment sources (upland soils and mass wasting sites) and stream bank samples using simple mixing models to determine sediment sources. Cesium-137 concentrations in the different geomorphic sources varied with upland soils > colluvial slopes > floodplain deposits > stream banks > Bay sediments. Preliminary results indicate that stream banks are probably the most significant sources of sediments deposited on the floodplains and in the Bay.

Last Modified: 8/2/2014
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