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This review of studies of the flooded areas of the Volga Delta (VD) ( F tot ) and the water balance (WB) of the VD during floods is a base of accumulated knowledge about the features of F tot and WB during floods and the methods of their study. The advantages and disadvantages of these methods are analyzed. It is revealed that the studies of F tot and WB during floods are not complete; only approximate daily F tot and approximate daily WB parameters for some floods were obtained. Therefore, the features of F tot and WB during floods of different water contents have not been fully identified. Research methods have changed and improved; hence, the accuracy of the approximate daily F tot and approximate daily WB parameters estimated by different methods for different floods vary significantly. The results of such calculations cannot be analyzed jointly. It will be impossible to supplement future studies with high accuracy with the results of such calculations. However, due to the appearance of new technologies and a large amount of satellite data, the methods can be implemented in the latest studies, obtaining highly accurate results. As a result, the following problems remain unsolved: calculating daily F tot and daily WB parameters for floods of different water content with high accuracy using high-tech methods with a large number of space images and identifying spatiotemporal patterns and distinctive features of floods of different water content by daily F tot and daily WB parameters.

The first find of the stone moroko (Pseudorasbora parva), an alien fish species in the Volga delta, in the western segment of the avandelta is reported. A juvenile specimen (total body length of 25.0 mm, standard length of 20.5 mm) was caught on April 27, 2024, on the southern borderline of the Damchiksky security zone.

Many geological sections along the Lower Volga valley have been actively studied since the end of the 19th century. The results of field work showed that in all the studied Quaternary sections along the Lower Volga valley, alluvial deposits of the Khvalynian time (the end of the Late Pleistocene) are absent. Geomorphological mapping was carried out and many outcrops along the Volga valley and on the Baer knolls were studied. In the Khvalynian time, the Volga most likely did not flow in its present position. The purpose of this article was to determine the locations of the paleochannels of the Volga on the basis of geomorphological data, in particular, the spatial location and morphology of the Baer knolls. A large multi-branched delta (about 200–210-km wide), probably similar in size to the modern Lena delta, was located significantly to the north of the modern one. The delta existed in the form of a series of small and large branches; on the site of the modern Volga-Akhtuba there was probably a small central branch, the Sarpa branch was located in the west, and the Elton-Khaki branch was located in the east. At the beginning of the Holocene, the Sarpa and Khaksky branches began to die off, and most of the water began to rush along the central branch, forming the modern Volga-Akhtuba valley. The Sarpa-Davan channel system continued to be active in the period after the formation of the Baer knoll complex, that is, at the beginning of the Holocene during the Mangyshlak regression.

New palynological and ostracod data are presented from the Holocene Volga delta, obtained from short cores and surface samples collected in the Damchik region, near Astrakhan, Russian Federation in the northern Caspian Sea. Four phases of delta deposition are recognized and constrained by accelerated mass spectrometry (AMS) radiocarbon ages. Palynological records show that erosive channels, dunes (Baer hills) and inter-dune lakes were present during the period 11,500–8900 cal. BP at the time of the Mangyshlak Caspian lowstand. The period 8900–3770 cal. BP was characterized regionally by extensive steppe vegetation, with forest present at times with warmer, more humid climates, and with halophytic and xerophytic vegetation present at times of drought. The period 3770–2080 cal. BP was a time of active delta deposition, with forest or woodland close to the delta, indicating relatively warm and humid climates and variable Caspian Sea levels. From 2080 cal. BP to the present-day, aquatic pollen is frequent in highstand intervals and herbaceous pollen and fungal hyphae frequent in lowstand intervals. Soils and incised valley sediments are associated with the regional Derbent regression and may be time-equivalent with the ‘Medieval Warm Period’. Fungal spores are an indicator of erosional or aeolian processes, whereas fungal hyphae are associated with soil formation. Freshwater algae, ostracods and dinocysts indicate mainly freshwater conditions during the Holocene with minor brackish influences. Dinocysts present include Spiniferites cruciformis, Caspidinium rugosum, Impagidinium caspienense and Pterocysta cruciformis, the latter a new record for the Caspian Sea. The Holocene Volga delta is a partial analogue for the much larger oil and gas bearing Mio-Pliocene palaeo-Volga delta.

The Volga delta is a large, dynamically changing water object. Its regime is determined by both natural water balance components and artificial streamflow regulation by the Volga hydroelectric power plant. The paper provides a brief overview of the studies of the Volga delta delta water regime and runoff distribution among the Volga delta branches, including those dealing with the runoff parametrization and computation. It also discusses the works on modeling the runoff distribution. The HEC-RAS software has been used to develop a one-dimensional hydrodynamic model of the Volga delta from its top (the Volga/Verkhnelebyazh’e gauging station) to the Astrakhan–Krasnyi Yar gauge line. The model calibration has been performed using in situ measurements in the delta in 2019 and network gauging data, the validation has been carried out based on the network gauging data for 2015, 2017, and 2018. Modeling results have shown that the contribution of runoff flowing to the Buzan branch source has slightly increased as compared to the period of 2001–2012.

From 1995 to the present, the level of the Caspian Sea decreased by almost 3 m, which significantly affects the morphometry and the ecological state of shallow water areas. First of all, this concerns the shallow Northern Caspian and, especially, the Volga Delta, which over the years has moved 10–20 km forward and grown by about 3000 km 2 in area. Changes in the morphometric characteristics of the Volga Delta inevitably affect the hydrodynamic, hydrophysical, hydrochemical, and hydrobiological characteristics of river water in the foredelta. From May 18 to 20,  2021, complex hydrophysical, hydrochemical, and hydrobiological studies of river water were carried out in the southwestern part of the Volga Delta. A total of 32 stations were completed, at which water surface temperature, electrical conductivity, turbidity, and chlorophyll a and oxygen concentrations were measured and samples were taken for further analysis in the laboratory. They showed a complex picture of the distribution of the studied parameters, both along the watercourses and across them. Only high spatial resolution optical and infrared Landsat-8 OLI/TIRS satellite images, obtained on May 20, 2021, almost simultaneously with the ongoing studies in the Volga Delta, helped the spatial distribution of these parameters to be understood. As a result of the analysis of the images, it was possible for the first time to identify a horizontal fine structure of watercourses, which is an alternation of clear immiscible narrow jets with significantly different characteristics of temperature, suspended matter, turbidity, and chlorophyll  a .

Lychagin, M.Yu.; Tkachenko, A.N.; Kasimov, N.S., and Kroonenberg, S.B., 2015. Heavy metals in the water, plants, and bottom sediments of the Volga River mouth area. Deltaic landscapes occupy a special place in environmental geochemical studies of water bodies. Their ecotone position at the land–sea boundary determines the biological richness of the deltaic ecosystems and creates a unique variety of landscape–geochemical conditions. The Volga River delta is one of the largest in the world. It serves as a geochemical “focus” that reflects the character of fluxes of substances of natural and anthropogenic origin in the entire system of the Volga basin. This paper presents the results of long-term environmental geochemical research of aquatic landscapes in the mouth area of the Volga River. The study has shown the high spatial and temporal variability of the chemical element migration in the area due to the influence of hydrodynamic conditions, variations in water and sediment discharge, and local features of the aquatic systems that determine their geochemical identity. The environmental geochemical conditions of the Volga delta are still relatively safe, especially compared with many other estuaries of the major rivers. Contamination of aquatic systems is manifested mainly in excess of heavy metals (Cu, Pb, and Cd) in suspended matter over the global background values, most notably in the flood period. The content of heavy metals in the water and sediments during the most recent decades remains low; pollution of the bottom sediments is largely insignificant and local.

On the basis of long-term (1995–2010 and 2013–2014) microbiological monitoring of water and pike perch in the various water areas of the Volga Delta, four dominant groups of bacteria from the family Enterobacteriaceae and genera Aeromonas, Flavobacterium , and Pseudomonas were identified. The results of studies on the species composition of water microflora and fish from natural populations and artificial breeding are summarized. The prevalence of potentially pathogenic microflora in the water and fish over the indicative ones has been shown. Water temperature influences the seasonal cycles of microorganisms and their pathogenicity factors, which determine their fast adaptation to living in different ecological niches.

The conditions of biogenic sedimentation (concentration of chlorophyll a (chl a ), particulate organic carbon (POC), and its isotope composition (δ 13 С POC )), as well as the quantitative characteristics of phytoplankton, and the total concentration of suspended particulate matter have been studied in the Caspian Sea in May and June 2012. The vertical (from the surface layer to bottom) distribution and precipitation of the biogenic component of suspended particulate matter have been determined. It was found that only 5% of the particulate matter and around 3% of POC reached the Middle Caspian after passing through a marginal filter (Volga River delta–Northern Caspian). The subsurface chl a maximum layer (around 20–60 m) with a cold-water phytoplankton community was revealed in the Middle and Southern Caspian. The subsurface region of accumulation of chl a and phytoplankton in the subthermocline layer has been extended from the southern periphery of the Derbent Depression to the Absheron Sill.