Journal of Coastal Research 25 2 265–272 West Palm Beach, Florida March 2009 Causes of Salt Marsh Erosion in Galveston Bay, Texas Thomas M. Ravens † *, Robert C. Thomas ‡ , Kimberly A. Roberts † , and Peter H. Santschi † ** † Texas A&M University at Galveston Galveston, TX 77550, U.S.A. santschi@tamug.edu ‡ HDR Inc. 555 North Carancahua Suite 1650 Corpus Christi, TX 78478, U.S.A. ABSTRACT RAVENS, T.M.; THOMAS, R.C.; ROBERTS, K.A., and SANTSCHI, P.H., 2009. Causes of salt marsh erosion in Gal- veston Bay, Texas. Journal of Coastal Research, 25(2), 265–272. West Palm Beach (Florida), ISSN 0749-0208. There is major salt marsh loss in Galveston Bay and other estuarine environments. In Galveston Bay, the causes of marsh loss include wave action, subsidence, eustatic sea-level rise, and insufficient sediment supply. To assess the relative importance of these factors in marshes of West Galveston Bay, wave action, sediment supply, and sedimen- tation rates were studied. Analysis of the data indicated a significant gap between the historic sediment accretion rate of 0.20 cm y -1 and the relative sea-level rise (the rate of rise of the water depth due to the combined effects of eustatic rise and subsidence) of 0.65 cm y -1 . Furthermore, in 94% of the eroding marshes, where 20% exceedance wave height was less than 0.17 m, the role of wave-induced erosion was relatively small. Thus, the major cause for salt marsh loss is insufficient sediment supply. These findings indicate that in the many eroding marshes in Galveston Bay, where wave action is not the major cause of marsh loss, marsh restoration efforts need to enhance sedimentation rather than wave protection. ADDITIONAL INDEX WORDS: Galveston Bay, subsidence, sedimentation, wetland loss, Cs-137, Pb-210. INTRODUCTION Between 1950 and 1989, about 12% of the salt marshes of Galveston Bay were lost due to subsidence, eustatic sea-level rise, wave action, and filling (White et al., 1993). Recently, a number of these eroding marshes have been subject to marsh restoration efforts that include the construction of a break- water to minimize wave impact as well as marsh grass plan- tations (Shiner, Moseley and Associates, 1998). However, these efforts did not include an accounting of sediment flows into the marsh. The Galveston Island State Park (GSP) marsh (Figure 1) is an example of a restoration project in West Galveston Bay on the north side of Galveston Island (Figure 2). Other res- toration projects on the north side of Galveston Island include Jumbile Cove, Delehide Cove, and Starvation Cove. These projects are all within 10 km of the Galveston Island State Park (Figure 2). The Galveston Island State Park marsh lost 405 of its original 445 hectare area between 1930 and 1994 (Glass and Hollingsworth, 1999). A geotube breakwater and marsh terracing, followed by planting, were done to ‘‘restore’’ the marsh. Marsh terraces are ridges that are created by dredging nearby sediment. The terraces are designed to pro- vide a large amount of edge (i.e., interface between land and water), which enhances marsh productivity (Rozas and Mi- nello, 2001). Various studies of marsh loss have been conducted in the DOI: 10.2112/07-0942.1 received 16 September 2007; accepted in re- vision 11 February 2008. * Present address: School of Engineering, University of Alaska An- chorage, 3211 Providence Drive, Anchorage, AK 99508, U.S.A. ** Corresponding author. Galveston Bay area and elsewhere. In the fluvial-deltaic marshes in the Trinity River Valley (adjacent to northern Galveston Bay, Figure 2), marshes have been inundated be- cause of insufficient sediment supply compared to the rela- tive sea-level rise (White, Morton, and Holmes, 2002). They measured an average sedimentation rate of 0.5 cm y -1 , which was significantly less than the local relative sea-level rise rate (1.1 cm y -1 ). Sediment supply might have been reduced because of damming of the Trinity River (e.g., K.W. Warnken and P.H. Santschi, unpublished data). Similarly, Yeager et al. (2007), radiometrically dating sediments from a wetland in northwest Houston, documented a sediment accretion rate of only 0.16 cm y -1 , significantly less than relative sea-level rise rates of more than 1 cm y -1 in this area. In studies of European salt marshes, increased marsh inundation frequen- cy due to channel dredging has been shown to affect marsh vegetation succession and also to cause marsh erosion (Thom- son et al., 2004). In Louisiana, studies have documented the reduction of ocean sulfuric acid in marshes. The hydrogen sulfide produced is thought to negatively impact marsh veg- etation, perhaps contributing to marsh loss (DeLaune, Smith, and Patrick, 1983). In Galveston Bay, salt marsh loss results from subsidence due to geologic faults activated by underground liquid ex- traction (White and Morton, 1997). At the same time, some salt marsh has been created because of the subsidence-in- duced conversion of uplands to wetlands (White et al., 1993). Wave action has been shown to be an important cause of marsh erosion in Galveston Bay (Shafer et al., 2003) and else- where (Roland and Douglass, 2005; Schwimmer, 2001). Sha- fer et al. (2003) found that marshes were stable if the 20% exceedance wave height (H 20% , the wave height exceeded 20%