River planform change downstream of the Sinclair Dam, Oconee River, Georgia

The Sinclair Dam on the Oconee River in Central Georgia was built in 1953 to generate electricity and has acted as a sediment trap most likely creating post-dam stream flows that were largely reduced of much of the pre-dam sediment loads. The dam is not used to practice flood control. The section of the Oconee River that is the focus of the research for this dissertation is located in both the Piedmont province with its more resistant igneous and metamorphic rocks and narrow, steep sided valleys and the more level Coastal Plain comprising wider, gentler valleys with highly developed floodplains. The length of the Oconee River that comprises the study area is 75 km. Spatial data were obtained from eight decades of aerial photographs that covered periods before and after the closure of the dam. Methods of data extraction and geospatial analysis involved the use of GIS and spatial adjustments of images using ERDAS Imagine.;It is hypothesized that the closure of the dam has caused the river downstream of the dam to be more stable through a reduction in the movement or changes in the planform of the river across its floodplain. The measurement of the river planform changes have determined that closure of the dam has led to a reduction of the lateral movement of the river over the floodplain, creating more stable conditions.;There is also a comparative analysis of the planform change of the Oconee of the two geologic provinces. Analysis shows that differences do exist in the rates of planform behavior of the river in the two geologic provinces. Furthermore, geological influence upon the Oconee River in the Piedmont province is limited with the riverine characteristics strikingly similar to the alluvial channel found in the Coastal Plain province. The evidence strongly supports the idea that river planform change follows a cycle of adjustment ranging between low to high lateral movements across the landscape.;There is also a discussion of meander migration of river and how this may be explained by the meander geometry of the curves along the river's course. The ratio of the radius value of the meander bend and the width of the curve are important variables in determining the degree of lateral and down-valley movement of the meanders themselves. The literature shows that maximum lateral movement of meanders occurred at rc/w ratios between 2 & 3. Findings of this study show that while 63% of all meanders show the maximum lateral migration rate occurring when the rc/w ratios fall between 2.64 and 2.92, maximum lateral movements of the channel between photographic years commonly occur outside of this optimal rc/w ratio of 2 & 3 proving that meander geometry does not fully explain the rates of channel migration. Furthermore, this approach does not explain the variability of ratios that exist from year to year throughout the data. The theory of organized criticality was tested with the available data and this found limited use in explaining why three of the ten existing cutoffs occurred after the critical threshold sinuosity was exceeded.;Very little work has been done on the effects of dam closure on lateral migration rates. This study is quite useful because it is based upon information that covers over four decades of data on stream channel migration after the closure of a dam, and just over two decades of data before the closure of the dam. Although only some theories pertaining to fluvial geomorphology were tested, there is, owing to the richness of the data, the possibility to test other theories that explain river channel behavior across landscapes.;Dams upon rivers are common features within the US and they will continue their influence upon the way rivers move across the landscape. Rivers undermine man-made structures such as bridges, road embankments, retaining walls, etc. They remove large sections of land as they create new land as they chart new courses in the course of their existence. Rivers therefore challenge engineers, land managers, property owners and scientists alike since they are generally not static with a tendency to remain in one position within the floodplain, but fluid with a propensity to change positions over time. This project is quite useful since it demonstrates that theories cannot always provide the best explanations for river channel movement across the landscapes. This study demonstrates that rivers the uniqueness of each river makes it difficult to provide a suitably comprehensive model that explains how rivers behave over time.