HISTORIC SHORELINE CHANGE AND SEDIMENTARY PROCESSES IN MUGU LAGOON-BARRIER SYSTEM, CA

GIS mapping and analysis of the mean high water shoreline location, combined with surveyed topographic profiles, were used to monitor changes in shoreline position in the Mugu Lagoon/barrier complex for the period 1857-1998. GIS source data includes historic aerial photos, topographic maps and bathymetric charts, all georeferenced to 1998 data. The shoreline displays dynamic equilibrium or landward retreat from 1857 until the 1940's, suggesting that sediment supply did not increase in response to rapid growth in agricultural development in local and regional watersheds. Rapid coastal accretion beginning 1940's is associated with construction of military harbor facilities at Port Hueneme located updrift of Mugu, and locally within the Point Mugu Naval Air Station. The Port Hueneme harbor later formed a littoral barrier that corresponds with rapid net erosion on the entire Mugu coastline. Beach renourishment south of Port Hueneme briefly reversed this trend in the 1960's, but since 1972, the much of this shoreline has been eroding more rapidly than at any time since 1857. The interior lagoon shoreline shows little net change between 1857 and 1901, after which the central and eastern lagoon began rapidly infilling. Local dredging and infilling briefly interrupted this trend between 1945 and 1970, after which infilling accelerates significantly in the central portion of the lagoon. Since 1970 these local impacts have been reduced substantially, and lagoon filling has primarily controlled by watershed scale processes. The pattern and timing of lagoon infilling correlates well with land use changes in the upland watershed. Between 1945 and 1990 agricultural acreage in the upland watershed was reduced by about one third, largely due to rapid expansion of suburban land use. Time series analysis of precipitation and stream hydrographs show a sudden increase in total annual discharge on watershed streams which deliver sediment to Mugu Lagoon beginning about 1970, corresponding with previous studies documenting measured increases sediment yield triggered by changing land use in this watershed.