River Delta Morphodynamics: Examples From the Danube Delta

The Danube, with its mouths at the Black Sea, has been economically and strategically one of the most important rivers in Europe; consequently, its delta has been studied since the Nineteenth Century. Although many morphological and sedimentological aspects of the Danube delta are well understood, its late Quaternary evolution remains ambiguous. This uncertainty reflects in part the complexity of the sea-level variations and water-chemistry changes related to the periodic isolation of the Black Sea during eustatic lowstands, but also a lack of accurate age control of the deltaic deposits. On the basis of a review of existing radiocarbon dates, we propose that the development of the delta at the open coast started approximately 6,000–5,500 14C years ago, much later than the 9,000 14C years BP previously suggested. Morphodynamics of the open-coast Danube delta has been determined largely by the interaction between fluvial deposition and the strong southward wave-induced longshore transport. Morphological and facies asymmetry displayed by the marine lobes of the Danube delta indicate that a strong and sustained southward-directed longshore transport has been a persistent process along the delta shore. Coastal evolution on the adjacent nondeltaic coast is also strongly coupled to the delta morphodynamics via the longshore transport. Analysis of recent deltaic progradation of the youngest open-coast lobes of Danube delta indicates that river-mouth morphodynamics is highly nonlinear, involving multiple feedbacks between subaerial deltaic progradation, deposition on the subaqueous delta, current and wave hydrodynamics as well as wave–current interactions. First, a feedback loop is activated by the hydraulic groin effect of the river plume, which leads to a mutually sustained progradation of the updrift coast and subaqueous delta at the mouth. Second, the development of a shallow subaerial delta platform, strongly offset to the downdrift direction, helps dissipate waves reaching the platform, leading to entrapment of sediment on the platform. Third, increased flood-induced deposition on the subaqueous delta platform, followed by wave reworking, leads to recurrent emergence of barrier islands at its offshore edge; longshore transport is then channeled (i.e., intensified and guided) by the new coast along the barrier, leading to a rapid alongshore expansion of the subaqueous delta in the downdrift direction. Although the sedimentation processes are complex, the resulting morphology at the mouth exhibits a tendency to self-organize that is reflected and preserved by the facies architecture of wave-influenced lobes. River Deltas—Concepts, Models, and Examples SEPM Special Publication No. 83, Copyright © 2005 SEPM (Society for Sedimentary Geology), ISBN 1-56576-113-8, p. xxx–yyy. INTRODUCTION The morphodynamics of deltaic coasts has been a less well explored aspect in the evolution of deltas. Coastal morphological units such as barriers, estuaries, or deltas have traditionally been studied in isolation, ignoring the fact they co-evolve interdependently (e.g., Cowell et al., 2004). Feedbacks between the evolving morphology of a delta and basinal hydrodynamics remain inadequately known, as are the interactions between contemporaneous lobes within a delta (e.g., Komar, 1973, 1998) or the influence exerted by a delta on the dynamics of adjacent nondeltaic coasts and vice versa (e.g., Penland and Suter, 1989; Jimenez et al., 1997; Stanley et al., 1997; Aslan et al., 2003; Corregiari et al., this volume). Although there has been much recent progress in understanding suspended-sediment deposition from river plumes (e.g., Syvitski and Bahr, 2001; Geyer et al., 2004), the dynamics of coarse sediments at river mouths has received little attention, in large part because of the inability of conventional techniques to provide reliable direct measurements. However, it is becoming increasingly clear that river-mouth processes involving bed-load transport exert a primary control on the larger delta morphology and facies architecture (e.g., Wright, 1985; Dominguez et al., 1983, 1987; Giosan, 1998; Rodriguez et al., 2000; Bhattacharya and Giosan, 2003, Fielding et al., this volume; Willis, this volume). Our paper explores the morphodynamics of the wave-influenced Danube delta with the intent to clarify its Holocene evolution. Although monitored since the middle of the Nineteenth Century, mainly for navigation purposes, many aspects of the evolution of the Danube delta, including a reliable chronology, remain uncertain. We critically review less accessible literature on the subject as well as discuss new data to outline a coherent model for the development of the Danube delta. We further argue that a large-scale morphodynamics approach is necessary for establishing key controls not only for the evolution of any delta plain but also for the adjacent nondeltaic coast and subaqueous delta. This is particularly important for the management of Danube delta, Europe’s largest wetland ecosystem, where economic and environmental interests among and within riparian nations are often divergent, leading to environmental pressure both from upstream factors as well as downstream changes in the Black Sea basin (Margesson, 1997; Lancelot et al., 2002). ENVIRONMENTAL SETTING