Vodnye resursy

The characteristics of the Onega River flow regime and its elements at the top of the mouth area over 80 years are presented. The analysis of the long-term and seasonal runoff variability is carried out, the characteristic of the intra-annual distribution were given. Statistical processing of long-term series of maximum discharges of spring floods and rain floods, minimum low-water discharges of the summer-autumn and winter periods was carried out. Patterns of long-term fluctuations of the annual and seasonal runoff and extreme water discharges have been revealed. The analysis of changes in runoff elements was based on identifying general long-term trends and comparing calculated parameters by periods, identified using difference-integral curves and chronological graphs. The features of fluctuations in Onega River flow, related with climate change started in the 1980-s are noted. Particular attention was paid to the water flow regime in the 21st century, characterized by a rising of its parameters.

Possible scenarios for the impact of climate change on hydrodynamic processes in the mouths of the Northern Dvina, Onega and Pechora Rivers are considered. For this purpose, we used hydrodynamic models of estuary areas previously developed by the authors in the STREAM 2D, HEC-RAS and Delft3D software packages. Projected sea level rise values, obtained from data of the global climate models GFDL-ESM2M, IPSL-CM5A-LR, HadGEM2-ES, MIROC-ESM-CHEM, were used to established the lower boundary condition of hydrodynamic models.

As the upper boundary conditions of hydrodynamic models, runoff change estimations performed using the ECOMAG runoff formation model were taken into account. Based on ensemble simulations, two main tendencies in changes in the factors of hydrodynamics of estuarine areas were identified and analyzed - sea level rise and river flow reduction.

Scenarios of spring flood and summer-autumn low water periods were modeled, corresponding to different combinations of boundary conditions for the historical period, as well as for the most probable and most unfavorable hydrometeorological conditions of the mid-21st century. An analysis of possible changes in the hydrodynamic characteristics of the water flow and their impact on various aspects of hydro-ecological safety and use of water resources in the estuarine areas of the rivers under study is presented.

The delta of the Kuban River, the largest river in the North Caucasus, formed under the influence of significant sea level fluctuations in the Holocene. The entire delta region of the Kuban River is divided into two parts – the Azov-Sea and the Black-Sea. The Black-Sea part of the marine edge of the Kuban River delta, represented by the Bugazskaya and Vityazevskaya bay bars separated by the Blagoveshchensky outlier, is a part of the lithodynamic system of the Anapa bay bar. The lithodynamic aspects of the development of the Black-Sea part of the of the Kuban River delta coastline (which largely determined the formation and dynamics of the Anapa bay bar) are analyzed based on field and remote sensing data, as well as archival and literary sources. It is shown that the alluvium of the Kuban River is a component of the sediment budget of the lithodynamic system of the Anapa bay bar during regression periods, in which solid runoff entered directly into the coastal zone of the Black Sea. During the Phanagorian regression, ~94 million m³ of tractional sediments could have been carried into the Black Sea coastal zone, which is comparable to the volume of the active sediment layer of the modern Anapa bay bar. During transgressions, sediments were deposited in the depths of sea bays, and after their separation from the sea by a bar (the future Anapa bay bar), of lagoons and did not reach the seashore. Since the beginning of the 20th century, under the effect of a combination of natural and technogenic factors, the flow of the Kuban River into the Black Sea has virtually ceased. Along the Bugayskaya and Vityazevskaya bay bars, areas with different rates of retreat of the seashore line predominate due to the increasing deficit of sediments in the lithodynamic system. This section of the coast can be considered the Kuban River delta coastline only in the paleogeographic sense.

A decrease of freshwater flow of the Don River during the period of severe low water flow observed in recent years, the growth of salinity of the Sea of Azov and Taganrog Bay, and sea level rise are accompanied by an increase in the number and intensity of dangerous salt water intrusions into the Don Delta. Retrospective analysis of the inflow of salt water into the delta shows an increase in salinity at the same levels of water rise during surges. This is especially clearly manifested in the lowest water years, when the frontal zone of the salinity gradient in Taganrog Bay approaches the delta coastline. The results of the analysis of the penetration range of salt water into the branches of the Don Delta allow us to state that changes in natural conditions have led to an increase in the distance to which salt water penetrates by 2–3 times compared to the 20th century.

In 2023, during the main phases of the water regime, four expeditions were carried out to collect water samples from sources to mouths in the Kamchatka and Avacha rivers, flowing into Kamchatka Bay and Avacha Bay of Eastern Kamchatka, respectively. In the r. In Kamchatka, seasonal extremes of concentrations and fluxes with river runoff of dissolved and organic phosphorus, as well as organic carbon, were established during the period of snow melting in the river valley and in the highlands (May–June), which was less in the river. Avacha. The annual fluxes with the runoff of the Kamchatka River and the Avacha River were respectively 4565 and 289 tons/year for Ptot, 9526 and 2006 tons/year for Ntot, 60485 and 3992 tons/year for DOC. The article discusses the sources and fluxes of nutrients in the catchments of the studied rivers and the potential responses of the aquatic ecosystems of the receiving basins in connection with volcanism, including in connection with the eruption of the Shiveluch volcano, which occurred on April 11, 2023.