Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Texas General Land Office Coastal Issues Conference, Corpus Christi, Texas, March 10-12, 2004
Preliminary Analysis of Wetlands Status and Trends on Galveston Island
Rachel L. Waldinger, William A. White, and Thomas A. Tremblay, and Thomas R. Calnan
Wetland loss in the northern Gulf of Mexico has been attributed to many processes, including subsidence, sea-level rise, faulting, and human activities. Extensive losses of marsh in the Galveston Bay system, including Galveston Island, were documented in 1993 by the Bureau of Economic Geology (BEG) as part of the Galveston Bay National Estuary Program. In these earlier studies, historical changes in wetlands were analyzed from the mid-1950's through 1989. In the present study, wetlands mapped using 2002 aerial photographs of Galveston Island were compared to maps from 1979 and the mid-1950's to determine rate and location of wetland change. Our analysis shows that the island's total estuarine emergent wetlands (salt and brackish marshes) have decreased in area over time and their spatial distribution has changed.
The 2002 wetlands map was interpreted and mapped in a GIS program from georeferenced color infrared aerial photographs. Units were subdivided into marshes, flats, water, and upland areas and assigned a water regime and other special modifiers, as specified by National Wetlands Inventory (NWI) guidelines. Areas where interpretations of wetland classifications were uncertain were field checked and verified. U.S. Fish and Wildlife Service produced the 1950's and 1979 maps, which also follow NWI guidelines. Wetland changes were quantified by comparing the three maps using GIS software and performing overlay analyses.
Preliminary overlay analysis shows that for both periods of time (mid-1950's through 1979 and 1979 through 2002), estuarine marsh loss exceeded marsh gain (Figure 1). However, rate of marsh loss slowed considerably during the later period of time, from a rate of approximately 19 ha/yr to 10 ha/yr. Preliminary examination of the habitats that are replacing marsh shows that in both time periods tidal flats and open water (including marsh that has been severely fragmented by water encroachment) replaced the largest percentage of marshes (53% or 872 ha in the first time period and 44% or 421 ha in the second). Most expansion in estuarine marshes came at the expense of upland units (45% or 541 ha in the first time period and 39% or 278 ha in the second) and estuarine flats (26% or 308 ha in the first time period and 32% or 230 ha in the second).
There are several reasons for changes in wetland distribution on Galveston Island determined by this study. Among the most important reasons for marsh loss was subsidence, which is in agreement with a previous BEG study. Subsidence of Galveston Island appears to have been caused primarily by groundwater pumping in the Texas City area (USGS studies). A tide gauge on Galveston Island has shown relative sea level to be rising over the long term at an average rate of 0.62 cm/year (NOAA, National Ocean Service), but over the short term, from the mid-1950's through the mid-1960's, at 1.9 cm/yr. Rates in the Texas City area, which is located at the center of a subsidence bowl, were much higher (USGS studies). As land subsides and relative water level rises, marshes are flooded and begin to be replaced by tidal flats and open water. Upland areas become influenced by tides and transition into marsh. Since Texas City switched to surface water and began water conservation measures (in the late 1970's), rate of subsidence has been significantly reduced (USGS studies). This reduction helps explain why marsh loss was greatest during the period from the 1950's through 1979. Lowered rates of subsidence have caused wetland change rates to slow. Other factors that contributed to wetland changes were housing development, which led to losses, and differences in interpretation of the aerial photographs, which led to apparent local losses and gains.