Keynote Presentation: Tom Dunne

Budgets of Lowland Floodplain Sedimentation

Understanding the sediment budgets of large, lowland rivers illuminates various aspects of their form and behavior, such as their planforms, rates of migration, and floodplain form. These budgets often reflect the interaction of fluid mechanics and sediment transport in the current hydroclimatic regime with crustal deformation and other slow, enduring, Earth processes. The small gradients are significantly perturbed by tectonic deformation of resistant materials beneath or along their valleys, causing changes to sediment transport capacities and channel migration. The effects of Quaternary sea-level changes propagated far inland, leaving a strong imprint on the modern budget of sediment transport and resulting alluvial landforms, and slow changes of land and sea level continue to affect the sediment balance and form of some river mouths. The scale of their channel and floodplain changes involves very large fluxes of sediment, and therefore the evidence of changes in their boundary conditions, such as sea level and sediment supply, persists for long periods of time. Hydrologic regimes along uncontrolled rivers force flow over bank for relatively long periods of time, and this regime combined with the typically fine sediment load of lowland rivers causes large amounts of overbank sedimentation. The combination of large amounts of overbank sedimentation with high channel shifting rates along many large rivers focuses attention on the interaction of channel and floodplain to explain the sediment budget of the valley.

The scale and complexity of these processes require the employment of various forms of monitoring from satellites, and the use of isotopes and hydrologic modeling, along with sediment sampling to understand the sediment budget of alluvial valleys. These concepts are illustrated with examples from the Amazon River basin in Bolivia and Brazil.