Abstract:

Bayhead deltas host large expanses of coastal wetlands, which provide myriad ecosystem services including serving as critical and protective buffers against storms and floodwaters, providing a variety of hydrological benefits like filtering and infiltration of water into the groundwater system, and importantly, providing critical nursery and habitat for many aquatic and terrestrial organisms. Most bayhead deltas along the Texas coast have experienced significant degradation and erosional loss over the last few decades, which marks a change in trend as many were growing and expanding during historic times. Sea level rise (SLR) is one of the primary controlling factors that moderates bayhead delta health and that of other coastal ecosystems, too. During the early Holocene about 10,000 years ago, melting of continental ice masses led to high rates of SLR along the Texas coast. These elevated rates of SLR prevented the majority of coastal ecosystems and environments that we know today from becoming established and growing. Only over the last few thousand years as rates of SLR slowed, and the relative roles of sediment supply and other factors increased accordingly, were these coastal ecosystems and environments able to stabilize and slowly develop into the systems we know today. Given current and future projected trends for SLR, which are approaching and will eventually exceed rates seen during the early Holocene, there is concern that our bayhead delta systems are at the tipping point of catastrophic retreat. To understand likely changes in these systems over the next sixty years, we used the Sea Level Affecting Marshes Model (SLAMM) to model potential wetland and landcover changes for the Trinity, Lavaca-Navidad, Guadalupe, and Nueces bayhead delta systems at three different time steps in the future (2040, 2060, and 2080), and under three different projected SLR scenarios including 1.0, 1.5, and 2.0 m of global mean SLR by 2100. The four systems show different potential responses and landcover changes ranging from gradual, to accelerating, to threshold like under different SLR scenarios. Model results for the Lavaca-Navidad system show the greatest susceptibility to change among all the systems. For example, open water composes about 29% of the total areal extent of the Lavaca-Navidad system at present day. Under the 1.0, 1.5, and 2.0 m of global mean SLR by 2100 scenarios mentioned above, SLAMM projects that by 2080, open water in the system will increase to 44%, 57%, and 65%, respectively. Another observation is that under higher SLR scenarios, several of the bayhead delta systems show a potential convergence in the areal percentages of landcover classes, suggesting that higher SLR rates might drive the systems towards a common evolutionary model. Overall, these model results help provide coastal resource planners and managers, decision makers, and the general public with up-to-date information about the status of our bayhead delta systems and their potential outlook for the near future. In general, model usage can better inform decision-making processes to make the most of finite and limited resources.