The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad.
This award will support a twenty-four month research fellowship by Dr. Amanda Keen-Zebert to work with Dr. Stephen Tooth at the University of Wales, Aberystwyth in the UK.
Wetlands are a focus of conservation efforts worldwide. However, geomorphological wetland research has been limited. In the Highveld of South Africa, the relationship between alluvial and bedrock river processes has consequences on the stability of wetlands that are an important water resource in a subhumid to semiarid area. The researchers in this project are investigating the long term development and sedimentological factors that effect wetland development and stability in South Africa. The researchers are adding a chronological element to a conceptual model of wetland development in the region that suggests that the progression from exhumation to breeching of dolerite intrusions into less resistant sedimentary rock controls the cycle of wetland development. By determining the bedrock incision rates through the dolerite intrusions using cosmogenic isotope analysis (CIA) and the timing of floodplain development through optically stimulated luminescence (OSL), it is possible to add long term chronological constraint to the conceptual model of wetland development. The improvement of the model will increase the understanding of river development in southern Africa, as well as the understanding of alluvial bedrock rivers and the interaction between alluvial and bedrock processes.
Current wetland rehabilitation efforts in southern Africa are directed at human causes of degradation. If the degradation of these wetlands is structurally geomorphic in nature, then management strategies need adjustment. The information gained in this study will benefit wetland conservation and rehabilitation programs in South Africa.
In the Highveld region of South Africa, variation in landscape resistance to erosion by rivers has consequences on the stability of wetlands. This research defines the long term development and sedimentological factors that affect wetland development and stability. The outcomes of this work are: 1. A defined chronology of the historical rates of variation in wetland formation in the South African Highveld. 2. Characterization of the spatial patterns in timing and process of floodplain formation in confined and unconfined bedrock channels 3. Illustration of the factors influencing the production and preservation of sedimentary archives. 4. First cosmogenic dataset on South African dolerite that will allow for interpretations of river incision. The results of this project indicate that the wetland sediments in the South African Highveld have been building for approximately 100 ka. This is quite stable compared to other sedimentary archive deposits from rivers worldwide. Processes operating in the wetlands, such as lateral channel migration, are very slow. Channel migration in the study rivers is ~0.16-0.33 m/yr. Wetland stability is dependent on geological factors, mainly the resistance to erosion of downstream reaches. Once channel incision or down-cutting is initiated in the wetlands, they degrade and dry out in less than 1ka, relatively fast compared to the long time period of wetland formation. Building weirs and other modifications that affect the hydrologic processes in the wetlands increases the rate of channel change. Natural channel processes take 103-104 years to recover from these types of interference. There is a large amount of variation in the spatial patterns of floodplain formation between river reaches in confined valleys and those in unconfined valleys. In unconfined reaches, floodplains form over 104-105 years. Sediment reworking is very slow due to low slopes and stream powers which represent the amount of energy a river can expend on its banks and bed. In confined reaches, stream power and slopes are higher and sediments are reworked or completely eroded and later refilled over 103 years. The depositional age of floodplain sediments in confined reaches range from 102-103 years. In the Highveld, the preservation of fluvial sedimentary archives is largely dependent on stream power and whether the valley is confined or unconfined. Data resulting from cosmogenic work on the dolerites in the confined reaches in the Highveld is the first of its kind. This dataset will allow interpretations of river incision rates which control the stability of the wetlands over long timescales. Because South Africa is tectonically quiescent, rivers play a major role in establishing the rate of landscape formation. Future work that builds on this research will define the rate of river incision which will elucidate the timing of landscape formation in the region.
