『Abstract
　The five largest rivers in East and Southeast Asia (Yellow, Yangtze, Pearl, Red and Mekong) are important contributors of terrigenous sediment to the western Pacific Ocean. Although they have annually delivered 〜2000×10⁹ kg of sediment to the ocean since 1000 yr BP, they presently contribute only 〜600×10⁹ kg/yr, which is reverting to a level typical of the relatively undisturbed watersheds before the rise in human activities in East and Southeast Asia at 2000 yr BP. During the most recent decades flow regulation by dams and sediment entrapment by reservoirs, as well as human-influenced soil erosion in the river basins, have sharply reduced the sediment delivered from the large river basins to the ocean. We constructed a time series of data on annual water discharges and sediment fluxes from these large rivers to the western Pacific Ocean covering the period 1950-2008. These data indicate that the short-term (interannual scale) variation of sediment flux is dominated by natural climatic oscillations such as the El Nino（後のnの頭に〜）/La Nina（後のnの頭に〜） cycle and that anthropogenic causes involving dams and land use control the long-term (decadal scale) decrease in sediment flux to the ocean. In contrast to the relatively slow historical increase in sediment flux during the period 2000-1000 yr BP, the recent sediment flux has been decreased at an accelerating rate over centennial scales. The alterations of these large river systems by both natural and anthropogenic forcing present severe environmental challenges in the coastal ocean, including the sinking of deltas and declines in coastal wetland areas due to the decreasing sediment supply. Our work thus provides a regional perspective on the large river-derived sediment flux to the ocean over millennial and decadal scales, which will be important for understanding and managing the present and future trends of delivery of terrigenous sediment to the ocean in the context of global change.

Keywords: River sediment flux; East and Southeast Asia; Western Pacific Ocean; Dams and reservoirs; Climate change』

Contents
1. Introduction
2. Regional setting
　2.1. Regional geography and topography
　2.2. Climatic setting
　2.3. Sources of river water and sediment
　2.4. Human activities in the river basins
3. Data collection and quality assessment
4. Results
　4.1. Yellow River
　4.2. Yangtze River
　4.3. Pearl River
　4.4. Red River
　4.5. Mekong River
5. Discussion
　5.1. Natural and anthropogenic impacts on sediment flux on the western Pacific Ocean
　5.2. Sediment delivery from millennial to decadal scale
　5.3. Impacts of decreased sediment flux on the coastal ocean
6. Conclusions
Acknowledgements
References

Fig. 1. Regional map of East and Southeast Asia showing the five major rivers that deliver sediment to the western Pacific Ocean. Fig. 2. Yellow River Basin: (A) major reservoirs and gauging stations on the mainstream, and time-series data (1950-2008) of annual sediment load recorded at stations of Huayuankou, Lijin (B) and at station Toudaoguai (C). Topographic relief of the river basin is produced based on the datasets of Shuttle Radar Topography Mission (SRTM), available at　http://dds.cr.usgs.gov/srtm/. The area surrounded by dashed line in panel (A) indicates the loess region (data from Durr et al., 2005). Major tributaries in panel (A) are: T1.Weihe　River, T2.Jinghe River, T3.Beiluohe River, T4.Yanhe River, T5.Wudinghe River, T6.Kuyehe River and T7.Huangfuchuan River. The gray bars in panels (B) and (C) indicate the　starting time of major reservoirs in the Yellow River basin. Fig. 5. Yangtze River Basin: (A) major reservoirs and gauging stations and (B) time-series data (1950s-2008) of annual sediment load recorded at major stations. Since 2002 the sediment loads at Hankou and Datong have become higher than at Yichang, indicating notable sediment supply from the middle and lower channel due to bed scour. Topographic relief of the river basin is produced based on the datasets of Shuttle Radar Topography Mission (SRTM), available at http://dds.cr.usgs.gov/srtm/. Fig. 7. Pearl River Basin: (A) major reservoirs and gauging stations, (B) time-series data (1950s.2008) of annual sediment load recorded at stations Gaoyao and Qianjiang, and (C) double-mass plot of cumulative water discharge and cumulative sediment load at station Gaoyao. Topographic relief of the river basin is produced based on the datasets of Shuttle Radar Topography Mission (SRTM), available at http://dds.cr.usgs.gov/srtm/. Fig. 8. Red River Basin: (A) major reservoirs and gauging stations, (B) time-series data (1960.2008) of annual sediment load recorded at station Son Tay, and (C) double-mass plot of cumulative water discharge and cumulative sediment load at station Son Tay. Topographic relief of the river basin is produced based on the datasets of Shuttle Radar Topography Mission (SRTM), available at http://dds.cr.usgs.gov/srtm/. Fig. 9. Mekong River Basin: (A) constructed and proposed reservoirs and gauging stations, (B) time-series data (1950s.2003) of sediment loads at stations of Gajiu and Jiuzhou in the upper Mekong River and at station Mukdahan in the lower Mekong River, and (C) double-mass plot of cumulative water discharge and cumulative sediment load at station Gajiu. Topographic relief of the river basin is produced based on the datasets of Shuttle Radar Topography Mission (SRTM), available at http://dds.cr.usgs.gov/srtm/. Fig. 13. Decadal variations in proportion of (A) water discharge and (B) sediment flux to the western Pacific Ocean from five major rivers in East and Southeast Asia. Fig. 14. Change in total sediment flux from the five major rivers to the western Pacific Ocean at the millennial scale (6000 yr BP to 1950) and decadal scale (1950.2008), illustrating historical increases and recent decreases due to human interventions in the river basins. Sediment flux data at millennial scale are from Milliman et al. (1987), Xue (1993), Ren and Zhu (1994), Saito et al. (2001), Ta et al. (2002a), Tanabe et al. (2006), Wang et al. (2007), Tamura et al. (2009), Zong et al. (2009), and Wang et al. (2010c). Fig. 15. Overview of sediment flux from major rivers in East and Southeast Asia to the western Pacific Ocean. “M&S (1992)” refer to the estimates from Milliman and Syvitski (1992), based on the dataset from the 1950s to the 1970s. Wang et al.（2011）による『Recent changes of sediment flux to the western Pacific Ocean from major rivers in East and Southeast Asia』から