Historical shoreline changes in the Galveston Bay and San Antonio Bay systems, Texas Gulf Coast.
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Changes in shoreline position and stability in the Galveston and San Antonio Bay systems since the late 1800's were documented using historical monitoring techniques. This is accomplished by comparing shorelines from topographic charts (dated 1850 to 1869) and aerial photographs (taken in 199-37, 1956-57, 1974, and 1982), measuring the magnitude (distance) of shoreline movement at specific sites, and calculating the rates of change for particular time periods (late 1800's to 1930's, 1930's to 1982, and late 1800's to 1982). Geological interpretations of the maps and photographs are used in conjunction with meteorological data and historical records to explain the important trends revealed in the tabulated shoreline data. Unprotected sediments forming the margins of these bays are subjected to natural coastal processes and modified by human activities that together cause shoreline movement. These unstabilized shorelines include high, nearly vertical clay bluffs, moderate slopes composed mainly of sand, salt-water marshes, sand and shell beaches, and newly formed areas filled by dredged material. Composition of the shoreline material and orientation of the shoreline with respect to prevailing wind directions and wave fetch largely determine the response and consequent movement of the shoreline. In some areas, property owners have attempted to stabilize the shoreline and prevent further movement by building seawalls and bulkheads and using riprap to dissipate wave energy. Factors contributing to shoreline changes include (1) regional and worldwide climate, (2) local changes in relative sea-level position, (3) local alterations insediment supply, (4) storm frequency and intensity, and (5) human activities. Historical data compiled for these various factors indicate that warming temperatures, rising sea level, decreasing sediment supply, recurring severe storms, and ongoing human activities all favor continued erosion of most unprotected bay shorelines.