Geomorphology of Mugu Lagoon 1857 to Present, Ventura County, California
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Mugu Lagoon is a large and relatively undisturbed estuary located at the mouth of Calleguas Creek on the Ventura County coast. Estuaries are rare in California as a result of high shoreline relief, arid climate, and human development. Mugu Lagoon has been protected from development by the U.S. Navy for 50 years and so is an excellent site to examine geomorphic changes caused by development of the watershed. This study examines changes over the period of 1857 to the present. Historic geographic data was extrapolated by tracing and digitizing nautical and topographic maps and air photos. This coverage was georeferenced by data point occupation using a GPS rover and then projected into real world coordinates using a GIS. Using the GIS, the data is analyzed by creating 143 years of maps and coverages illustrating the accretion and erosion of sediment. Pronounced geomorphic changes in Mugu Lagoon clearly correlate with both natural and human events. Historic storms and fires produce changes in sediment erosion and accretion. Changes from undisturbed natural land to agricultural and urban land resulted in beach sediment starvation and infilling of the lagoon interior in the Mugu Complex. Channelization of Calleguas Creek drastically decreased the sediment load delivered to the Mugu area and caused erosion of the beach. Mugu Lagoon is a fragile and dynamic system that is constantly adjusting to both natural and human variables. As long as the variables keep changing the dynamic equilibrium that characterizes Mugu Lagoon will also keep changing. To restore the dikes to an orientation predicted by one of the above models, at least 90 degrees of clockwise rotation would be required. Paleomagnetic data from the Quottoon pluton (58 Ma) reveal an average of only 45 degrees of counterclockwise rotation about a vertical axis. This rotation is similar to that described by Symons (1977) as the Hawkesbury Warp. In addition, preliminary paleomagnetic evidence indicates that the dikes have not been rotated. The orogen-parallel extension represented by the dikes cannot be fully explained using present models. The absence of dike rotation implies that the Hawkesbury Warp formed between 58 Ma and the Miocene.