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 among subaerial deltaic progradation, deposition on the subaqueous delta, current and wave hydrodynamics, and 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 subaqueous 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.