Organic Matter Characteristics and Variability of Flood Deposited Alluvial Sediments from a Small Mountainous River in the Oregon Coast Range.

Soils that develop on river deposited sediments are some of the most productive for crops and forests. Understanding the organic composition of the deposits is important in terms of soil C cycling, soil N cycling, and productivity. This study aims to characterize flood deposits along the Alsea River in the Oregon Coast Range. Sediments were deposited by a flood occurring during the Fall of 2007 and collected within 3 days of the peak flow. These deposits were fractionated by size into coarse particles (CP; > 63 µm) and fine particles (FP; <63 µm). All fractions were analyzed for organic C and N concentration, lignin, and cutin. Whole sediment samples were analyzed for surface area, and radionuclides (²¹⁰Pb, ¹³⁷Cs, and ⁷Be). Analyses to date show that there is negative relationship with specific surface area, C and N concentration and river mile. This effect is largely due to sorting by the river. There is also a slight negative trend in C concentration of the fine particles with river mile which may also be caused by sorting of particles by density as opposed to size. This has implications in that particles that have the greatest potential to affect productivity are deposited nearer the ocean and estuary.