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The pressure control of the anthropogenic factors has consequences on the fluvial systems, generating structural changes. The hydrology, hydraulics and sediment load of the rectified meandering systems depend on the hydrologic connectivity with the main stem of the river. The former meanders can be permanently, temporarily disconnected with the main channel producing interruptions in the transfer of fresh water and thus affecting the morpho-sedimentological processes and the biodiversity. This paper aims to investigate the distribution of the water and suspended sediment fluxes between the former meanders and the artificial man-made canals along the St. George distributary and thus, the hydrological connectivity. Understanding these complex human pressures are of high importance for reaching/maintaining the ecological status of the Danube. Herein, we made investigations along the St. George branch on many sub-systems river-channel-lake site type, formed by cutoff meanders, connective channels and lakes to observe how much the water and sediment inputs to the delta depressions is affected by the structural changes of the meander's physiography. [ABSTRACT FROM AUTHOR]

Microplastics represent one of the most important concerns of the actual society. With a long environmental persistence, these plastic based particles are transported by the flow and deposited in sedimentary systems. This paper describes the distribution of microplastics in fluvial and lacustrine environments of the Danube Delta. Taking into account that the River Danube and its main distributaries (Kilia, Sulina and St. George) are the primary pathways along which MPs are transferred to the Black Sea, the superficial sedimentary material could testify the pollution degree. Microplastic concentrations identified previously by Pojar et al. (2021a) and the data of flow velocities and fluxes at different hydrological regimes were considered to characterise the MPs distribution, as their abundance and morphology change due to the discharge and currents speed. Our results show that moderate to higher concentrations have been found in the first and second rank locations, related closely with the main distributaries. Complementary, we discuss how hydrodynamics can elucidate the spatial distribution of MPs in the Danube Delta, as well as their path and behaviour in a regional-scale aquatic environment. [ABSTRACT FROM AUTHOR]

Major (CaCO3, TOC, and Fe2O3), minor (MnO and TiO2) and trace (Zr, Cr, V, Zn, Cu, Ni, As, Pb, and Hg) elements from surface riverbed sediments were determined at 66 locations in deltaic environment, along St. George distributary and in the interdistributary depressions of the Danube Delta. The studied area is characterized by terrigeneous, either non-carbonated or low calcareous sediments, quite poor in organic matter. Most of the studied heavy metals showed high spatial variability, generally with higher concentrations in interdistributary depressions and adjacent canals, as well as on the meander Mahmudia (M1). Ni, Cu, and Cr showed concentrations frequently exceeding the quality criteria set in the Romanian legislation. Contamination Factor (CF) and Pollution Load Index (PLI) were used to assess the level of heavy metal pollution in the studied area. [ABSTRACT FROM AUTHOR]

In the Danube Delta, on the Sulina branch, the morphology, sediment, and bedform characteristics were investigated. Three-dimensional (3D) bathymetry, flow velocity, suspended-load concentration, and liquid and solid discharge data were acquired throughout several cross sections along the Sulina channel, in order to investigate the distribution of water and sediment discharges and their influence against the river bed. A single observation (in February 2007) was made regarding the geometry, sediment composition, and hydraulic conditions under which the dunes grew and degenerated. The investigation focuses here mostly on the geometrical parameters of these bedforms, such as height, length, as well as grain size characteristic of the sediment and water dynamics. Based on in-site measurements, different hydraulic parameters were calculated, such as bed shear stresses and Reynolds number. During the field campaign, the measured water mean velocity was from v = 0.22-1.13 ms-1. At the same time, the measured range of shear stresses within the dune field formation was from π0 = 2.86 N·m-2 (on the cutoffs) to 8.62 N·m-2 (on the main channel). It was found that the correlation between height (H) and length (L) of the Sulina branch dunes describes the formula: H = 0.093L0.5268. The bedforms of the Sulina channel are, in general, developed in fine sand (D50 between 0.06 and 0.35 mm). [ABSTRACT FROM AUTHOR]