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The Bolivian Andes flank one of Earth's major topographic features and dominate sediment input into the Amazon Basin. Millennial-scale erosion rates and dominant controls on erosion patterns in this range are poorly known. To define these patterns, we present 48 erosion rate estimates, derived from analysis of in situ 10 Be in quartz-bearing alluvium collected from the Upper Beni River basin.Erosion rates, corrected for the non-uniform distribution of quartz in the sample basins, range from 0·04 mm a ). Hence, our data do not record any significant variation in erosion rate over the last several million years. Mean and modal short-term erosion rates for the Andes are an order of magnitude lower than rates in the Ganges River headwaters in the High Himalaya and an order of magnitude greater than rates typical of the European Alps.In the Upper Beni River region of the Bolivian Andes, short-term, basin-averaged erosion rates correlate with normalized channel steepness index, a metric of relative channel gradient corrected for drainage area. Neither normalized channel steepness index nor basin-averaged erosion rate shows strong correlation with mean basin hillslope gradient or mean basin local relief because many hillslopes in the Upper Beni River region are at threshold values of slope and local relief. Patterns of normalized channel steepness index appear primarily to reflect tectonic patterns and transient adjustment to those patterns by channel networks. Climate and lithology do not appear to exert first-order controls on patterns of basin-averaged erosion rates in the Bolivian Andes.

Continental-scale rivers with a sandy bed sequester a significant proportion of their sediment load in flood plains. The spatial extent and depths of such deposits have been described, and flood-plain accumulation has been determined at decadal timescales, but it has not been possible to identify discrete events or to resolve deposition on near-annual timescales. Here we analyse (210)Pb activity profiles from sediment cores taken in the pristine Beni and Mamore river basins, which together comprise 720,000 km2 of the Amazon basin, to investigate sediment accumulation patterns in the Andean-Amazonian foreland. We find that in most locations, sediment stratigraphy is dominated by discrete packages of sediments of uniform age, which are typically 20-80 cm thick, with system-wide recurrence intervals of about 8 yr, indicating relatively rare episodic deposition events. Ocean temperature and stream flow records link these episodic events to rapidly rising floods associated with La Niña events, which debouch extraordinary volumes of sediments from the Andes. We conclude that transient processes driven by the El Niño/Southern Oscillation cycle control the formation of the Bolivian flood plains and modulate downstream delivery of sediments as well as associated carbon, nutrients and pollutants to the Amazon main stem.

The world's rivers deliver 19 billion tonnes of sediment to the coastal zone annually 1 , 17 with a significant fraction being sequestered in large deltas, home to over 500 million 18 people. Most (>70%) large deltas are under threat from a combination of rising sea 19 levels, ground surface subsidence and anthropogenic sediment trapping 2,3 , and a 20 sustainable supply of fluvial sediment is therefore critical in preventing deltas being 21 'drowned' by rising relative sea levels 2,3,4 . Here, we combine suspended sediment 22 load data from the Mekong River with hydrological model simulations to isolate the 23 role of tropical cyclones (TCs) in transmitting suspended sediment to one of the 24 world's great deltas. We demonstrate that spatial variations in the Mekong's 25 suspended sediment load are correlated (r = 0.765, p < 0.1) with observed variations 26 in TC climatology, and that a significant portion (32%) of the suspended sediment 27 load reaching the delta is delivered by runoff generated by TC-associated rainfall. 28Furthermore, we estimate that the suspended load to the delta has declined by 52.6 ± 29 explaining past 5,6,7 , and anticipating future 8,9 , declines in suspended sediment loads 34 reaching the world's major deltas. However, our study shows that changes in TC 35 climatology affect trends in fluvial suspended sediment loads and thus are also key to 36 fully assessing the risk posed to vulnerable coastal systems. 37 Mt over recent years (1981-2005The world's largest rivers contribute a disproportionately large fraction (Extended 38Data Table 1) of the terrestrial sediment flux, which has both created, and is critical in 39 sustaining, their great deltas. Moreover, river borne sediments are a key vector for carbon 40 and nutrients, thereby playing a vital role in global biogeochemical cycles 10,11 . However, a 41 significant majority (>70%) of large deltas are now recognized as being under severe 42 threat from rising relative sea levels 2,3 , in part due to reported anthropogenically-driven 43 reductions in sediment loads 5,6,7 . Many large rivers are located in tropical regions 44 (Extended Data Figure 1) that exhibit highly seasonal flow regimes affected by tropical 45 cyclones (TCs). The potential destructive or constructive impacts of tropical cyclones that 46 directly strike deltas are well established 12,13 . However, when they strike further upstream 47TCs deliver much higher than normal levels of rainfall, effectively triggering landslides 48 and mobilizing sediments into the river network, thereby generating very high 49 instantaneous sediment loads 14,15,16 . Such high sediment loads could compensate for the 50 potential destructive effects of TCs striking deltas proper but, notwithstanding some prior 51 studies in smaller drainage basins 17,18 , the role of TCs in driving sediment delivery to the 52 lowlands and coast remains unclear. As noted, this is particularly the case for large rivers 53 that carry much of the terrestrial sediment flux because these rivers are, in t...

A B S T R A C TTo predict erosion rates throughout the Andes, we conducted a multiple regression analysis of the sediment discharge from 47 drainage basins in the Bolivian Andes and various topographic, climatologic, and geologic parameters. These mountainous basins are typically large (17-81,000 km 2 ; k m 2 ), often have decades of measurement mean p 11,000 data on daily water and sediment discharge, and display an extraordinary range of denudation (0.01-6.9 mm/yr), runoff (16-2700 mm/yr), and local topographic relief (700-4300 m), yet the underlying lithology (granitic plutons, metasediments, and Quaternary deposits) can be classified into a small number of homogeneous types, and anthropogenic disturbance is limited. The steep nature of the channels precludes sediment storage, and unlike previous global studies of fluvial denudation rates, based on data compilations from very large river basins (1100,000 km 2 ), this analysis distinguishes the sediment production in mountains from sediment entrapment within adjacent sedimentary basins. Lithology and average catchment slope account for 90% of the variance in sediment yield, and yield is not significantly correlated with runoff. However, because runoff over geologic timescales orchestrates the processes of channel network incision and sediment evacuation, climate could ultimately govern basin hillslope conditions and thereby the rates of hillslope erosion. Several theoretical geomorphic models for mass wasting are tested to assess hillslope-scale sediment yield models for the study basins. When applied throughout the Amazonian Andes, such empirical models predict an annual Andean sediment flux to the lowland Amazon Basin of 2.3-3.1 Gt. Because ∼1.3 Gt/yr of sediment reach the gauged tributaries of the mainstem Amazon River, the intervening foreland basins appear to intercept about half of the total Andean sediment discharge.

Recent growth of the construction industry has fuelled demand for sand, with considerable volumes being extracted from the world's large rivers. Sediment transport from upstream naturally replenishes sediment stored in river beds, but the absence of sand flux data from large rivers inhibits assessment of the sustainability of ongoing sand mining. Here, we demonstrate that bedload (0.18 Mt yr-1 ± 0.07 Mt yr-1) is a small (1%) fraction of the total annual sediment load of the lower Mekong River. Even when considering suspended sand (6 Mt yr-1 ± 2 Mt), the total sand flux entering the Mekong delta (6.18 Mt yr-1 ± 2.01 Mt yr-1) is far less than current sand extraction rates (50 Mt yr-1). We show that at

The Anthropocene is proposed as a new interval of geological time in which human influence on Earth and its geological record dominates over natural processes. A major challenge in demarcating the Anthropocene is that the balance between human-influenced and natural processes varies over spatial and temporal scales owing to the inherent variability of both human activities (as associated with culture and modes of development) and natural drivers (e.g. tectonic activity and sea level variation). Against this backdrop, we consider how geomorphology might contribute towards the Anthropocene debate by focusing on human impact on aeolian, fluvial, cryospheric and coastal process domains, and how evidence of this impact is preserved in landforms and sedimentary records. We also consider the evidence for an explicitly anthropogenic geomorphology that includes artificial slopes and other human-created landforms. This provides the basis for discussing the theoretical and practical contributions that geomorphology can make to defining an Anthropocene stratigraphy. It is clear that the relevance of the Anthropocene concept varies considerably amongst different branches of geomorphology, depending on the history of human actions in different process domains. For example, evidence of human dominance is more widespread in fluvial and coastal records than in aeolian and cryospheric records, so geomorphologically the Anthropocene would inevitably comprise a highly diachronous lower boundary. Even to identify this lower boundary, research would need to focus on the disambiguation of human effects on geomorphological and sedimentological signatures. This would require robust data, derived from a combination of modelling and new empirical work rather than an arbitrary ?war of possible boundaries' associated with convenient, but disputed, ?golden? spikes. Rather than being drawn into stratigraphical debates, the primary concern of geomorphology should be with the investigation of processes and landform development, so providing the underpinning science for the study of this time of critical geological transition. Copyright ? 2016 John Wiley & Sons, Ltd.authorsversionPeer reviewe

[1] 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 210 Pb geochronology of floodplain cores from 11 transects to measure deposition rates over the past $65 years, finding a decline from 5.5 cm/a (0-10 m distance from the channel) to 0.9 cm/a ($400 m) 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 $318 river km we studied for the lowland Strickland, we calculate 12-19 Mt of annual sediment accumulation ($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 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 $10 km, this implies a waiting time of $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 ($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.Citation: Aalto, R., J. W. Lauer, and W. E. , Spatial and temporal dynamics of sediment accumulation and exchange along Strickland River floodplains (Papua New Guinea) over decadal-to-centennial timescales,