Influenced by both terrestrial and marine processes, deltas are critical sedimentary landforms at the land-sea interface that provide records of past environmental changes. Deltas worldwide are currently being severely altered by human activities, in particular river damming, which has reduced the amount of sediment reaching the coast. Although large dams are relatively recent phenomena, human alteration of landscapes has been ongoing since the advent and expansion of agriculture. Combining field data collection, fluvial modeling, and coastal evolution modeling, this research will examine whether human activities have significantly influenced the formation of the modern Ebro Delta, on the Mediterranean coast of Spain. Specifically, it is hypothersized that the growth of the Ebro Delta can be attributed to massive deforestation for agricultural purposes initiated in Roman times. Field research will document the evolution of the Ebro Delta over approximately the last 3,000 years by collecting and analyzing sediment cores from along the delta plain. The climate-driven hydrological model HydroTrend will be used to compute riverine sediment discharge to the coast over this time period, incorporating changes to sediment delivery that could be attributable to climatic changes and those that could be caused by human alterations to the Ebro River watershed. A numerical model of shoreline evolution capable of incorporating long timescales will be applied to examine the causes of the delta's shape and to evaluate how changes to riverine discharge would affect the coast after being reworked by waves. Coupling of the fluvial and coastal numerical models serves an important goal as proof-of-concept of the Community Surface Dynamics Modeling System (CSDMS).
Understanding the historic and future morphologic change of deltas has become increasingly important as sea levels rise and sediment loads feeding deltas continue to be sequestered behind dams in the hinterland. Deltas are often densely populated and provide many importantt natural resources to society, particularly along the rocky Spanish Mediterranean coast with scant land near sea level. In regions that have been affected by human civilization, deltas can serve as a record of climate and land-use changes across large watersheds. The Ebro Delta, Spain, with its distinctive plan-view shape, has experienced significant morphologic changes over the last two millennia; changes that may be attributable to human activities in the Ebro watershed. The results of this research will have wide applicability to unraveling the history of deltas worldwide (e.g., Nile, Po, Danube, Indus, Mississippi). If human activities are largely responsible for the shape of the world's deltas, this would provide an important context for understanding how humankind will manage these resources over the coming centuries, particularly as climate changes and humans continue to alter these landforms.
The Ebro Delta, Spain, with its distinctive plan-view shape suggestive of robust advance followed by rapid erosion, has experienced significant morphologic changes over the last two millennia—changes that may be attributable to anthropogenic activities, including land clearing, within the Ebro River watershed. Combining field research, fluvial modeling, and coastal evolution modeling, this research project addressed the working hypothesis that human activities have significantly influenced the shaping of the Ebro Delta and river deltas in general. The project investigated which aspects of a delta’s morphology and depositional history could be attributed to external (allogenic) forcing, such as human activities or centennial-to-millennial climate change, and which aspects could be attributed to background natural variability and autogenic mechanisms such as avulsion and reworking by waves. This project included field data investigations of the Ebro River delta and floodplain via drilling and coring as well as experiments using numerical models of shoreline evolution. Significant findings and accomplishments include the following: The angle of approaching waves can have a strong control on plan-view delta shape. Numerical modeling results demonstrate that wave angle plays a strong influence on delta evolution. For more broadly spread wave approach angles, a delta will prograde faster. An asymmetry in wave approach angle can result in asymmetric evolution of delta shape, but only if the delta shore protrudes significantly from the shore. (Ashton and Giosan, GRL 2011) The manner in which waves rework "abandoned" delta lobes can be predicted using pre-abandonment geometry and wave climate characteristics. The Coastline Evolution Model (CEM) was used to model the effects of drastic reduction of sediment delivery to delta lobes, finding that pre-abandonment plan-view geometry and wave angle climate control the shape of reworked abandoned lobes. Results suggest that development of alongshore-extending spits, such as those found on the Ebro, tends to occur only when a delta is close to fluvial dominance before sediment supply is eliminated. (Nienhuis et al., GRL 2013) We have contributed to the NSF Community Surface Dynamics Modeling System (CSDMS) by coupling the Coastline Evolution Model with Hydrotrend through the Component Modeling Tool (CMT). Coupled models are now available for public use through the CSDMS website. This modeling coupling allowed the investigation of the influence on delta morphology of changing river angle and bi-directional feedbacks between rivers and the delta shore. (Ashton et al., Computers and Geosceinces, 2013) The morphology of the Ebro delta changed radically over the last 2000 years. Using radiocarbon ages, morphostratigraphic information, literature surveys of climate and human impacts and historical cartography, an evolution model for Ebro delta has been generated and compared to the evolution of upstream adjacent floodplain. The morphology of the delta changed over the last two millennia from a small cuspate wave-dominated delta into a strongly river dominated multi-lobe delta before reverting to wave dominance in the last few centuries. External factors such as increasing land use and climate-modulated floods are largely responsible for these transformations (Giosan et al., in prep., 2014). Scenario-based modeling simulations suggest a significant change in sediment flux was necessary to develop the Ebro delta’s morphology. Using modern wave climates for the Ebro Delta region, model simulations show that a significant (more than two times) increase in sediment supply was necessary to change the delta from its cuspate shape to one that would develop recurved spits post-abandonment. The orientation of the delta spits is congruent with current wave climates and interactions between adjacent lobes has affected the development of these spits. The required sediment input is greater than that attributable to climate change alone. (Ashton et al., in prep, 2014). Overall, this research helps illuminate and constrain the hypothesis that the effects of human activities, such as land clearing, have a measureable effect on the shape of the Ebro Delta. Both sedimentary analyses and modeling results point to a significant change in the rate of sediment supply to the Ebro Delta starting approximately 2,000 years ago. Beyond the Ebro Delta itself, the project research developed quantitative understanding of the controls on the shape wave-influenced deltas, both during growth and after reduction in sediment supply. The project findings are especially relevant for a better management of future delta shoreline changes in response to anthropogenic influences (river damming) and climate change (change in discharge and wave climate).
