Abstract
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.
Biography
Tom Dunne was briefly mentored by Demonstrator Olav Slaymaker at Cambridge University before earning a Ph.D. from Johns Hopkins University. While working for the USDA Agricultural Research Service and McGill University Geography Department, he conducted research on the effects of topography, soil characteristics, and vegetation on runoff processes under rainfall and snowmelt in Vermont and Labrador. Teaching at the University of Nairobi, Kenya, he initiated an enduring research interest in Africa, including experimental studies of runoff and erosion, and statistical studies and field surveys of the effects of land use on hillslope erosion and basin sediment yields. He spent more than two decades in the Department of Geological Sciences at the University of Washington, enjoying annual visits with the UBC Hydrology group, and supervising research on landslides and debris flows; basin sediment budgets in natural and managed forests; tephra erosion and lahars on active volcanoes; and sediment transport and channel morphology in sand-bed and gravel-bed river channels. From his beachside office at the University of California at Santa Barbara, this hair-shirted product of glaciated, coal-bearing regions now studies hydrology, sediment transport, and floodplain sedimentation in the Amazon River of Brazil, the Andes Range and adjacent floodplains of eastern Bolivia, and the Sacramento River valley of central California. Geographia vos liberabit!