The evolution of an asymmetric deltaic lobe (Sf. Gheorghe, Danube) in association with cyclic development of the river-mouth bar: Long-term pattern and present adaptations to human-induced sediment depletion

Abstract

The asymmetric Sf. Gheorghe lobe is the only active lobe in the Danube delta associated with a river-mouth bar (and related barrier islands and spits) that has continuously displayed cyclic development during the last 1500years. During the early cycles, the Sf. Gheorghe distributary experienced a significant increase in sediment load (by an order of magnitude) as a result of the successive avulsions occurred 1.5–1.2ka, which redistributed more water and sediment captured from the neighboring branches of Împuţita (southern distributary of the Sulina arm) and Dunavăţ. Morphological and sedimentological analyses together with a newly obtained chronology throw light on the multiple ridgeset (10) structure of Sf. Gheorghe lobe, each of them (excepting the first one) following a common evolutionary pattern reflected by the cyclic succession of the recurring stages: i) subaqueous mouth bar building, ii) barrier island emergence, and iii) transformation into a barrier spit with several secondary spits. The spits become encased into the muddy deltaic plain as narrow sandy ridges building out on the downdrift side of the lobe as a barrier-marsh plain, whereas the updrift side constantly accreted fed by longshore currents, forming a classic beach ridge plain. The size of each ridgeset increased exponentially with every new cycle due to the constant lengthening of the coastline as the downdrift side of the lobe advanced seaward through a series of progressively larger similar quadrilaterals, yielding a corresponding geometric progression of the delta front size. Even though each newly formed ridgeset (cycle) had a longer lifespan (the latest cycles lasting 4–5 times longer than the first ones: 200–440years versus 50–80years), the evolutionary model remained unchanged as long as the balance between wave- and river-borne sediments, defined by the sedimentary index (Si), remained constantly low (Si≤0.1), whereas the mean advance rates of the river mouth remained constant at c.10m/year. Abrupt changes occurred within the last cycle (since the beginning of the 20th century) as a consequence of human-induced depletion of sediment supplied by the Danube flow, and is mainly expressed by the complete cessation of the updrift coastal progradation and the prevalence of erosion in front of the river mouth. These changes, which engendered a threefold increase of the Si (0.37 at present), are reflected by the recent (1930s–present) river mouth dynamics, characterized by cessation of its long-term seaward expansion in favor of downdrift migration. This is indicative of the transition of the Sf. Gheorghe mouth from an asymmetric to a deflected wave-influenced delta morphology whose current developments mark a significant change in the multicentennial cyclic evolutionary pattern. The asymmetric to deflected transition reflects the net increase in the influence of wave-driven sediment circulation on river mouth morphology, corresponding to a critical Si threshold of ca. 0.2 (≤0.2: asymmetric; >0.2: deflected).