『Abstract
　Cosmogenic nuclide-based denudation rates and published erosion rates from recent river gauging in the Napo River basin (Peruvian Amazonia) are used to decipher erosion and sedimentation processes along a 600 km long transect from the headwaters to the lowlands. The sediment-producing headwaters to the Napo floodplain are the volcanically active Ecuadorian Andes, which discharge sediment at a cosmogenic nuclide-based denudation rate of 0.49±0.12 mm/yr. This denudation rate was calculated from an average ¹⁰Be nuclide concentration of 2.2±0.5×10⁴ at/g(Qz) that was measured in bedload-derived quartz. Within the Napo lowlands, a significant drop in trunk stream ¹⁰Be nuclide concentrations relative to the Andean hinterland is recorded, with an average concentration of 1.2±0.5×10⁴ at/g(Qz). This nuclide concentration represents a mixture between the ¹⁰Be nuclide concentration of eroded floodplain deposits, and that of sediment eroded from the Andean hinterland that is now carried in the trunk stream. Evidence for addition of sediment from the floodplain to the trunk stream is provided by published decadal-scale sediment flux measurements from gauging stations operated in the Napo basin, from which an increase from 12×10⁶ t/yr at the outflow of the Andes to 〜47×10⁶ t/yr at the confluence with the Solimoes（後のoの頭に〜） (upper Amazon River) is recorded. Therefore, approximately 35×10⁶ t of floodplain sediment are added annually to the active Napo trunk stream. Combined with our nuclide concentration measurements, we can estimate that the eroded floodplain deposits yield a nuclide concentration of 〜0.9×10⁴ at g(Qz) only. Under steady state surface erosion conditions, this nuclide concentration would translate to a denudation rate of the floodplain of 〜0.47 mm/yr. However, we have no geomorphologic explanation for this high denudation rate within the low relief floodplain and thus suggest that this low-nuclide concentrated sediment is Andean-derived and would have been deposited in the floodplain at a time when erosion rates of the Andes were elevated. Therefore, the recently eroded floodplain sediment provides an Andean “paleo denudation rate” of 1.2 mm/yr that was calculated for high Andean production rates. A likely period for elevated erosion rates is the LGM, where climate and vegetation cover of the Andes differed from that of the Holocene. A possible cause for the erosion of the floodplain is the tectonic uplift of the Eastern Andes, which progressively shifts the Napo River northwards. Hence, the river cuts into ancient lowland sediment, which is admixed to the Andean sediment carried in the main Nap River.

Keywords: Erosion; Denudation; Cosmogenic beryllium-10; In situ-produced ¹⁰Be; River loads; Andes; Sediment transport; Napo basin; Oriente basin; Channel-floodplain exchange』

1. Introduction
　1.1. Previous work
2. Study area
3. Sampling, sample processing and methodology
　3.1. Treatment of gauging-derived sediment yields and cosmogenic nuclide-derived sediment fluxes in depositional settings
4. ¹⁰Be nuclide concentrations and corresponding denudation rates in the Napo River basin
　4.1. Cosmogenic nuclide concentrations
　4.2. Cosmogenic nuclide-derived denudation rates from in situ-produced ¹⁰Be
5. Discussion
　5.1. Comparison with a published gauging-derived sediment budget
　5.2. The decrease of cosmogenic nuclide concentrations within the floodplain
6. Conclusions
Acknowledgements
References