Patterns of sediment accumulation in the Siletz River Estuary, Oregon

Abstract

Anecdotal evidence suggests many Pacific Northwest estuaries are filling with sediment due to historical logging activities in upstream watersheds. Using the Siletz River estuary as a case study, this research began by analyzing timber harvest and discharge records of the Siletz River watershed, and found that increased timber harvest coincides with a period of higher discharge. Based on these findings, sediment flux from the Siletz watershed was expected to have increased and resulted in higher sediment accumulation rates (SARs) in the estuary. To test these assumptions, SARs were estimated using down-core profiles of excess 210Pb and 137Cs from thirty-three cores taken within the estuary. Digital x-radiographs and grain size distributions of cores were used to provide a timeframe for flood deposits and provided further information on retention-related functions of the system. Results indicate minimal evidence for changes in SARs with only two 210Pb profiles and six 137Cs profiles that indicate an increase in deposition attributable to land use and hydroclimatic changes. Calculated SARs (0.18cm/y) were comparable to the rate of local sea level rise (0.19 cm/y), which indicates that retention in the estuary is influenced by the available accommodation space. This finding helps explain the distinct difference in sediment supplied (5.91 x 107 kg/y) to, and retained (8.42 x 106 kg/y) in, the estuary. Overall, this study illuminates the complexity of the forces that influence sediment flux from a watershed and retention within an estuary.