Spatial and temporal dynamics of sediment accumulation and exchange along Strickland River floodplains (Papua New Guinea) over decadal-to-centennial timescales

We investigated the processes of sediment exchange between the Strickland River and its lowland floodplain, documenting (1) the rates, textures, and distributions of sediment accumulation, (2) temporal variations in these rates, (3) the remobilization of sediment by erosive processes, and (4) whether net storage is significant over century timescales. We used Pb-210 geochronology of floodplain cores from 11 transects to measure deposition rates over the past similar to 65 years, finding a decline from 5.5 cm/a (0-10 m distance from the channel) to 0.9 cm/a (similar to 400 m) to similar to 0.1 cm/a (>1 km). Rates are elevated along curved sections of channel and are stable over time, and episodic accumulation is prevalent and may be correlated to floods. Integrated temporally and spatially, the average rate is 1.6 cm/a. Integrating along both sides of similar to 318 river km we studied for the lowland Strickland, we calculate 12-19 Mt of annual sediment accumulation (similar to 0.05 Mt/km or 0.07% of the annual load per km of river length), representing 17-27% of the total annual sediment flux. We used georeferenced Landsat images (1972-2001) to quantify channel migration and the resulting return of sediment to the channel. Mean lateral migration was 5.1 +/- 0.8 m/a. Given the floodplain width of similar to 10 km, this implies a waiting time of similar to 1 ka between floodplain formation and subsequent reentrainment of the bank as the channel migrates laterally. This raises the possibility that the net return of material from the floodplain due to channel migration could balance the overbank deposition we observed. The exchange flux between cut bank erosion and point bar deposition is 20-40 Mt, highlighting the significance of sediment recycling (similar to 50% of the total load). Such sediment trapping and recycling affects the transport, storage, and evolution of biogeochemically reactive particles, the evolution of the floodplain, and the morphodynamics of basin infilling.