Slowly down the Yangtse: landscape denudation and sediment flow in China

Slowly down the Yangtse: landscape denudation and sediment flow in China

J. Chappell

Erosion, loss of soil and river degradation through the impacts of human activities are familiar problems. Although methods for improving the situation sometimes seem obvious, the real characteristics of many such problems are not accurately known because processes in natural landscapes were not assessed before human populations became large. Such processes are difficult to assess from short-term observations, because high-impact events such as very large floods occur rarely. However, much of the essential information is preserved in modern and ancient riverine floodplain deposits.

The Yangtse is one of the world's largest river systems. Rising in very high plateaus and mountains of north Tibet and western Sechuan, it passes through the lowlands of eastern China and delivers a large quantity of sediment to the East China Sea. Contrasting climates and sea levels between Quaternary glacials and interglacials

doubtless induced major changes of regional erosion and sedimentation. In collaboration with colleagues at Tongji University in Shanghai, the project aims to evaluate long-term rates of erosion and soil loss by measuring rare isotopes producedby cosmic rays entering surface rocks. Average erosion rates and the transport history of sediments can be gauged by measuring the concentration of these isotopes in hillslope deposits and river sand. The measurements are made by Dr Keith Fifield and his accelerator mass spectrometry (AMS) team in the Research School of Physical Sciences and Engineering here at ANU.

Our previous studies in various terrains have proven the method in small catchments (Annual Reports 2000; 2001) and provide the basis for using similar methods at the much larger scale of the Yangtse River system. In the first phase of the project, sand samples were collected throughout the Yangtse river and its major tributaries: results are used to test regional models of slope erosion and sediment transport that explore the effects of tectonic uplift, climate change and human impacts. Initial results based on cosmogenic 10Be in quartz grains from the sand samples (illustrated in Figure 9) indicate higher than expected erosion rates in mountain catchments in eastern China, pointing to high human impact in historical times.