Geochemical Characteristics of Overbank Deposits after a Flood Event in a Small, Mountainous River System in the Oregon Coast Range, USA

The geochemical characteristics of particulate organic matter (POM) transported by rivers has broad implications in our understanding of aquatic nutrient dynamics, the fate of contaminants, environmental change in watersheds, and carbon export to depositional environments. The major fraction of this POM is mobilized during storms, especially in small mountainous river systems (SMRS) producing complex spatial-temporal POM patterns poorly documented due to logistical difficulties. In this study, we examine the use of overbank flood deposits as a surrogate of a quasi-Lagrangian POM sampling scheme to supplement the conventional Eulerian sampling scheme for POM. We report on the geochemical characteristics of 11 overbank deposits created after a significant flood (10 X mean discharge) along 80 km in the Alsea River, a SMRS in the Oregon Coast Range. We measure organic carbon, nitrogen, stable isotopes, and biomarkers such as lignin-derived phenols as well as particle size distribution and surface area of the deposited sediments. We compared those characteristics with the POM sampled during several storms at a fixed location. Our results suggest that despite the differences in local depositional conditions inferred from particle size distributions and texture, the geochemical properties of overbank deposits resemble the properties of the material in transport, mainly derived from a terrestrial source with a clear signal of gymnosperm wood. Furthermore, the normalized ranges of the geochemical indicators measured across space for one single event are comparable to, or even higher than, the normalized range of the same indicators measured along time at the fixed location. The implications of the amount and quality of the additional information offered by the overbank deposits in POM dynamics in watershed is discussed.