Shoreline Changes on Matagorda Island and San Jose Island (Pass Cavallo to Aransas Pass), An Analysis of Historical Changes

Historical monitoring along Matagorda Island and San Jose Island records the nature and magnitude of changes in position of the shoreline and vegetation line and provides insight into the factors affecting those changes. Documentation of changes is accomplished by the compilation of shoreline and vegetation line position from topographic maps, aerial photographs, and coastal charts of various vintages. Comparison of shoreline position based on topographic charts (dated 1857-99) and aerial photographs (taken in 1931-37, 1957-58, 1965, and 1974) indicates short-term changes of accretion and erosion along Matagorda and San Jose Islands between Pass Cavallo and Aransas Pass. Erosion produces a net loss in land, whereas accretion produces a net gain in land. Comparison of the vegetation line based on the aforementioned aerial photographs indicates short-term cycles of erosion related to storms (primarily hurricanes) and recovery during intervening years of low storm incidence. Long-term trend or direction of shoreline changes averaged over the 117-year time period of this study indicates that net changes on Matagorda Island have been predominantly accretionary with two exceptions. Net erosion of 600 to 2,025 feet occurred in the vicinity of Pass Cavallo owing to reorientation of the shoreline; minor net erosion and accretion along the southernmost 2,500 feet of the island were influenced by the southern migration of Cedar Bayou. The remaining shoreline of Matagorda Island experienced net accretion of 25 to 1,050 feet; average net accretion was 333 feet. Net accretion on Matagorda Island was influenced largely by major accretion which occurred between 1857 and 1937. Net rates of accretion for the southern part of the island were generally less than 1 foot per year whereas net rates of accretion for the northern half increased from a minimum of 1.1 feet per year to a maximum of 9.1 feet per year. Net erosional rates in the vicinity of Pass Cavallo ranged from 5.1 to 17.3 feet per year. Net changes on San Jose Island indicate alternating shoreline segment of net accretion and net erosion. In general, net shoreline changes were 75 feet or less, suggesting relative shoreline stability. Extreme net accretion of 750 to 1,400 feet was recorded for the southern portion of San Jose Island where rapid accumulation of sediment occurred in the vicinity of the north jetty at Aransas Pass. Net rates of erosion and accretion on San Jose Island were generally less than 1.5 feet per year. Because of limitations imposed by the technique used, rates of change are subordinate to trends or direction of change. Furthermore, values determined for long-term net changes should be used in context. The values for rates of net change are adequate for describing long-term trends; however, rates of short-term changes may be of greater magnitude than rates of long-term changes, particularly in areas where both accretion and erosion have occurred. Major and minor factors affecting shoreline changes include: (1) climate, (2) storm frequency and intensity, (3) local and eustatic sea-level conditions, (4) sediment budget, and (5) human activities. The major factors affecting shoreline changes along the Texas Coast, including Matagorda and San Jose Islands, are relative sea-level rise, compactional subsidence, and a deficit in sediment supply. Changes in position of the vegetation line are primarily related to storms. Studies indicate that changes in shoreline and vegetation line on Matagorda Island and San Jose Island are largely the result of natural processes, perhaps expedited by man's activities. A basic comprehension of these physical processes and their effects is requisite to avoid or minimize physical and economic losses associated with development and use of the coast.