Investigating organic matter in Fanno Creek, Oregon, Part 2 of 3: Sources, sinks, and transport of organic matter with fine sediment

•Geomorphic mapping identified locations and mechanisms of erosion and deposition.•Erosion and deposition rates were determined from field measurements.•Total organic carbon content was less in bank material than in floodplain deposits.•Suspended sediment export was estimated from continuous turbidity and flow data.•OM from bank sediment is small compared to floodplain debris added by mass wasting. Organic matter (OM) is abundant in Fanno Creek, Oregon, USA, and has been tied to a variety of water-quality concerns, including periods of low dissolved oxygen downstream in the Tualatin River, Oregon. The key sources of OM in Fanno Creek and other Tualatin River tributaries have not been fully identified, although isotopic analyses from previous studies indicated a predominantly terrestrial source. This study investigates the role of fine sediment erosion and deposition (mechanisms and spatial patterns) in relation to OM transport. Geomorphic mapping within the Fanno Creek floodplain shows that a large portion (approximately 70%) of the banks are eroding or subject to erosion, likely as a result of the imbalance caused by anthropogenic alteration. Field measurements of long- and short-term bank erosion average 4.2cm/year and average measurements of deposition for the watershed are 4.8cm/year. The balance between average annual erosion and deposition indicates an export of 3,250metric tons (tonnes, t) of fine sediment to the Tualatin River—about twice the average annual export of 1,880t of sediment at a location 2.4km from the creek’s mouth calculated from suspended sediment load regressions from continuous turbidity data and suspended sediment samples. Carbon content from field samples of bank material, combined with fine sediment export rates, indicates that about 29–67t of carbon, or about 49–116t of OM, from bank sediment may be exported to the Tualatin River from Fanno Creek annually, an estimate that is a lower bound because it does not account for the mass wasting of organic-rich O and A soil horizons that enter the stream.