Master of Science (MS)
Geology and Geophysics
The Garonne River of southwestern France presents a unique opportunity to study the controls on long-term incision and terrace formation within a large-scale fluvial system. The Garonne heads in the Pyrenees, flows through the Aquitaine Basin, and discharges into Atlantic Ocean via the Gironde Estuary/Bay of Biscay. From field data, three terrace complexes were identified and traced for >251 km from the base of the Pyrenees to the onset of tidal influences. Each complex is separated from adjacent complexes by scarps >10 m and represent 10s to 100s kyrs during which time the river occupied a finite elevational range within its valley, and lateral migration and cutting of straths dominated over valley incision. Using incision rates determined from other European studies, the 3rd terrace complex likely formed 500-250 kyr BP, the 2nd terrace complex formed 200-100 kyr BP, and the 1st terrace complex formed 100-50 kyr BP. Without direct chronological control, it is not possible to infer correlation between periods of deposition or bedrock incision and specific climatic conditions. Following previous work, a numerical model was developed to test the dependence exerted by slope and discharge on incision. Hypotheses tested include detachment-limited, transport-limited, and total stream power relationships, as well as a null hypothesis where incision is independent of discharge and slope. Each model has a specific range of exponents for discharge and slope (m and n values, repsectively) within the overall incision equation. Error and fit for each model formulation were evaluated using statistical tests. Results from the detachment-limited and transport-limited models were all unacceptable, whereas the total stream power model produced three acceptable combinations of m and n. However, the best-fit result for this system is the null hypothesis. It is therefore concluded that the models tested in this study do not describe the hydraulics behind incision in this river, or valley incision is independent of slope and discharge. Future work should focus on refinement of models so as to test for differential uplift and variations in lithologic controls, and development of independent geochronological control so as to evaluate causal mechanisms.
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Lancaster, Robin Rene, "Fluvial evolution of the Garonne River, France: integrating field data with numerical simulations" (2005). LSU Master's Theses. 3351.
Michael D. Blum