The response of river mouths to large-scale variations in sea level and river runoff: Case study of rivers flowing into the Caspian Sea

Regularities in the response of the mouths of major rivers, flowing into the Caspian Sea, to large-scale variations in its level and river water runoff and sediment yield are considered. Changes in the morphological structure and hydrological regime of the Volga, Terek, Sulak, Ural, and Kura mouths have been analyzed in both geological past and separately for three modern periods: a considerable drop in Caspian Sea level before 1978, its abrupt rise in 1978–1995, and a relative stabilization in the subsequent years. Specific features were identified in the hydrological-morphological processes in different mouths, caused by the differences in river sediment yields, and the slopes of delta surface and mouth nearshore beds. Some theoretical and methodological approaches were verified in the analysis and evaluation of the processes under consideration. The obtained results of studies of the mouths of rivers flowing into the Caspian Sea can be regarded as examples and analogues in the assessment of processes, which take place at the mouths of other Russian and world rivers at present and can take place in the future under anticipated natural and anthropogenic variations in sea level and river runoff.     

Studying Long-term Variations in the Caspian Sea with the Use of the Theory of Stochastic Differential Equations

Nonlinear mechanisms of long-term variations in the Caspian Sea level are described. It is shown that with account taken of the dependence of the evaporation depth from the Volga basin surface on soil moisture content and the dependence of the evaporation depth from the sea surface on its level, we obtain a fundamentally new (chaotic) oscillation mechanism with several attraction levels. The stochastic differential equations describing the water budget of the sea basin and the sea proper and the respective solutions of the Fokker–Planck–Kolmogorov equation are shown to have stationary bimodal density of the level probability. The random process, characterizing the sea level variations at a nonlinear dependence between the evaporation rate and the level is found to be non-Gaussian. Noise-induced transitions, caused by nonlinear evaporation processes are described. A new nonlinear stochastic theory describing the Caspian Sea level variations and based on predicted physical effects is suggested.     

Nonlinear dynamics of level variations in the Caspian Sea

Caspian Sea level variations, which have several equilibrium states, are studied by the methods of the theory of nonlinear dynamic systems. Normal monthly values of sea level according to data collected at Makhachkala gauge from 1900 to 2000 are considered. The diagnostic characteristics of dynamic chaos are used to show that sea level variations have some properties of series with chaotic behavior. A model of level variations in the Caspian Sea, comprising a system of water balance equations for the sea basin, the dynamics of river runoff, and water balance of the sea itself, is proposed. Equation of a nonlinear oscillator is derived and shown to have solutions with chaotic regimes at some combinations of parameters.     

Nutrient Input into the Caspian Sea with River Runoff

Long-term observational data are used to compare and analyze time and space variations in the concentrations of nutrients in the water of major rivers flowing into the Caspian Sea and assess the nutrients runoff into the sea. Annual variations in the normal monthly values of river runoff and nutrient compound concentrations and input into the sea are considered (18 compounds and considered for the Volga, Ural, Terek, Sulak, and Samur, and 7 compounds are considered for the Kura). The Volga contribution to nutrient input into the sea is found to vary from 77 to 94% with the average of 86%.     

Sediment balance at river mouths and the formation of deltas at rising or falling sea level

Sediment balance at river mouths—a physical basis of delta dynamics—is considered. The specific features of relationships between sediment balance components at a mouth of a river are established for a stable, rising, or dropping sea level. The development of the delta of the Chilia branch at the Danube mouth is considered as an example of delta dynamics under the conditions of a relatively stable sea level. The evolution of the Sulak delta in the Caspian Sea with a highly variable level is considered as an example of delta dynamics in the case of a considerable rise and drop of water level in the water body. The anthropogenic reduction of sediment runoff of the rivers (by a factor of two in the Danube, and by a factor of nine in the Sulak) is taken into account. The relationship between the sediment runoff of a river and the volume of the “backwater prism” that formed due to sea level rise is shown to be the key factor in the development of delta in the case of sea level rise. In the case of a drop in the sea level, the relationship between the “active” and “passive” progradation of the delta into the sea is determined by the sediment runoff of the river, the rate of sea level drop, and the bed slope in the coastal area of the nearshore zone.     

Long-Term Water Level Variations in the Eastern Sea of Azov and in the Mouth Reach of the Don River

Studies of long-term water level variations at marine hydrometeorological stations in the eastern Sea of Azov established a rise in the sea level which accelerated in the past 40 years. Allowance for the tectonic component permitted assessing the average rate of eustatic rise in the level. Oppositely directed long-term level variations were established in the mouth area of the Don River. Water level was found to rise at the downstream gages because of the backwater effect caused by the Sea of Azov level rise and delta deposits subsidence and to drop at the upstream gages mainly because of bed erosion owing to a reduction in sediment runoff after the construction of the Tsimlyanskoe Reservoir.     

Hydrological and Morphological Processes in the Kura River Delta

Peculiarities of the development of the Kura Delta over the last 200 years are discussed. As shown, changes in the delta were greatly affected by the Caspian Sea level drop during 1929–1977 and its rise during 1978–1995 as well as by natural and human-induced variations in the water and sediment runoff of the river. It was noted that the delta area decreased by nearly 40% as a result of the sea level rise. The following significant changes in the Kura Delta were revealed in recent years using space images: river water rushed through the right mouth spit and, hence, the main Southeastern Branch was devoid of its flow and a new sea spit began its formation in the branch mouth using wave-cut products.     

Regularities in the spatial distribution of some macro-and microelements in the Caspian Sea water

The distribution of some microelements (Sr, Si, Ba, Mo, Rb, and Mn) in the surface and deep-sea waters in various parts of the Caspian Sea is considered in comparison with the distribution of salt-forming macroelements. The observed mean concentrations of microelements and their variations over the sea area and with depth indicate to the presence of additional, other than Volga runoff, sources supplying Sr, Ba, U, Rb, and Mn to the Caspian Sea.     

A hydrologic contribution to risk assessment for the Caspian Sea

The Caspian Sea (CS), the world's largest inland sea, may also be considered as large-scale limnic system. Due to strong fluctuations of its water level during the 20th century and the flooding of vast areas in a highly vulnerable coastal zone, economic and environmental risk potentials have to be considered. Since the major water input into the CS is attributed to the Volga river, the understanding of its long-term flow process is necessary for an appropriate risk assessment for the CS and its coastal area. Therefore, a top-down approach based on statistical analyses of long-term Volga flow series is pursued. For the series of annual mean flow (MQ) of the Volga river basin during the 20th century, a complex oscillation pattern was identified. Analyses for multiple gauges in the Volga river basin and Eurasian reference basins revealed that this oscillation pattern resulted from the superposition of oscillations with periods of ∼30 years (MQ) in the western part of the Volga river basin, and ∼14 years (flow volume of snowmelt events) and ∼20 years (flow volume of summer and autumn) in the eastern part of the Volga river basin (Kama river basin). Almost synchronous minima or maxima of these oscillations occurred just in the periods of substantial changes of the Caspian Sea level (CSL). It can thus be assumed that the described mechanism is fundamental for an understanding of the CSL development during the 20th century. Regarding the global climate change, it is still difficult to predict reliably the development of the CSL for the 21st century. Consequently, we suggest an ongoing, interdisciplinary research co-operation among climatology, hydrology, hydraulics, ecology and spatial data management.     

Shedding light on the timing of the largest Late Quaternary transgression of the Caspian Sea

The Late Quaternary history of the Caspian Sea remains controversial. One of the major disagreements in this debate concerns the stratigraphic correlation of various deposits in the Caspian Basin. In this paper we identify and date, for the first time, the Enotaevka regression, lying between the two major phases of the largest Late Quaternary Caspian Sea transgression, the Khvalynian transgressive epoch, and provide a minimum estimate of sea level decrease during this regression. The River Volga is the major source of water to the Caspian; the Lower Volga region is unique in its record of palaeogeographic events, and this provides the opportunity to build a single stratigraphic and palaeogeographic history for the Pleistocene of Central Eurasia. Here we use luminescence to establish a new chronology for the largest Late Quaternary transgressive epoch of the Caspian Sea. The existing radiocarbon chronology does not allow the resolution of the two transgressive phases of this epoch (Early and Late Khvalynian). Based on clear palaeontological and geomorphological evidence, these must be very different in age, but shells associated with both transgressions gave very scattered ages of between 8 and 50 ka. This ambiguity has led to considerable discussion concerning the existence or otherwise of a deep Enotaevka regression phase between the two Khvalynian transgressions. Recently we have again identified these deposits at Kosika, on the right valley side of the Volga River. The new luminescence chronology described here, based on quartz OSL and K-feldspar pIRIR₂₉₀ ages, allows us to reconstruct the complicated history of Late Quaternary sedimentation in the southern part of the Lower Volga valley. The Kosika section reflects the following major stages: (1) the earlier Khazarian transgressive epoch; (2) a decrease in the sea level with the development of a freshwater lake/lagoon in the Volga valley; and (3) the Khvalynian transgressive-regressive epoch, including both the Early and Late Khvalynian transgressive periods, and the intercalated Enotaevka regression. Sea level during the early stage of the Khvalynian transgression reached Kosika at about 23–22 ka (approx. −1 to −2 m asl). This event is of the same age as the “grey clay” strata at the base of the Leninsk section marine unit (Kurbanov et al., 2021), also formed at the beginning of the Early Khvalynian transgression. Around 15–14 ka the Khvalynian basin moved to a regressive stage, and in the northern part of the Lower Volga the top part of the well-known ‘Chocolate Clay’ accumulated. In the southern part of the valley marine accumulation stopped at about 12–13 ka. This allows us to reconstruct a decrease in Early Khvalynian basin sea level between 15–14 ka and 13–12 ka ago, of about ∼15 m. At the Kosika section sediments derived from the Enotayevka regression are visible as a weakly developed palaeosol with evidence of surficial erosion, and these sediments are now dated to 13–12 ka. At 8.6 ± 0.5 ka, during the period of the Mangyshlak regression, aeolian deflation processes reworked sediments deposited by immediately preceding Late Khvalynian transgression.     

Computer hydrodynamic model of the Lower Volga

The structure of a hydrodynamic model of the Lower Volga, which has been developed for solving water management problems, and some results of computer simulation of variations of water level at the reach from the Volgograd HPP to the Caspian Sea are considered. The conditions under which the one-dimensional hydrodynamic model, developed in Delft Hydraulic Laboratory of Technical University based on Saint-Venant equations, can be successfully applied in the Lower Volga are determined.     

Probabilistic Estimates of Water Level in the Ural River Mouth Reach at Interaction between the River Runoff and Positive Setups

A probabilistic technique is developed for assessing water level in the mouth reach of the Ural River in the course of interaction between the river runoff and irregular rises in the Caspian Sea level (positive setups) at the predicted background sea levels of –26.5 and –26.0 m abs.     

Studying Seasonal Processes in the Volga Mouth Area with the Use of Remote Sensing Data

Monitoring of the following hydrological processes was carried out: breakup in the mouth areas of rivers in the northern European Russia, the propagation of the spring flood wave and inundation in the Volga-Akhtuba floodplain and the Volga delta, a catastrophic flood in the Terek mouth area, and ice phenomena in the Volga mouth area and in the northern Caspian Sea. The results of satellite monitoring of seasonal processes in the Volga mouth area are presented.__________Translated from Vodnye Resursy, Vol. 32, No. 3, 2005, pp. 261–273.Original Russian Text Copyright © 2005 by Gorelits, Zemlyanov.     

Regime of water balance components of the Caspian Sea

The results of studying the hydrological regime of the Caspian Sea and its basin climate in observation period 1945–2010 are generalized. The results of analysis of the regime of precipitation, air temperature in the Caspian Sea basin and its level, as well as Volga runoff in periods of Caspian Sea level rise and drop are given. The conformity in variations of the trends in Caspian Sea level its basin climate is demonstrated, and the direction of further studies is substantiated.     

Long-term variations in river and sea factors responsible for the hydrological regime and morphological structure of the Thames River mouth area

General geographic features of the Thames River, its basin, and mouth area, consisting of the tidal mouth reach of the river, a large estuary, and an open nearshore zone of the river mouth (the North Sea coastal zone) are discussed. The peculiarities of river and sea hydrological factors responsible for the regime of the Thames River mouth area are described in detail. Characteristics of the river water runoff were specified and supplemented by the data on the river inundations in the area of London. Particular emphasis was placed on variations in the mean sea level in the area of the Thames River mouth as well as on specific features of tides and storm surges in the area of the sea inlet into the estuary. Main regularities in the estuary evolution during Holocene and present-day morphological processes in the Thames River mouth area were revealed.     

The nonstationary and nonlinear character of Caspian Sea level variations

A stochastic model of level variations in a drainless water body is considered. The model accounts for the nonstationary character of sea level under stationary climate. It is shown that in the case of the Caspian Sea, the range from ?29 to ?26 m BS corresponds to a relative stability of the level because of the greatest gentleness of the shores and shelf slope in combination with the maximal water outflow into Kara-Bogaz-Gol Gulf.     

Wind-induced Variations in Water Level in River Mouths

The principal factors that control the character of deformation variations in water level in mouth areas of rivers are considered. The role of wind regime and the sea level variations in the formation of positive and negative setups is assessed. River mouth zones are classified in terms of the predominance of positive or negative variations in water level. Specific features of the positive or negative setup regimes in the mouth zones of the Neva, Don, and Volga rivers are discussed.     

Analysis of Long-Term Variations in the Volga Annual Runoff

The stability of sample estimates of statistical parameters was analyzed for segments of the initial time series of annual runoff volumes of the Volga River at Volgograd for 1881/1882–1994/1995. The segments of series considered in this study differ in the extent of anthropogenic impact on the runoff and the type of atmospheric circulation and correspond to characteristic periods in the Caspian Sea level variations. The conclusion is made that there are statistically significant variations in the annual runoff of the Volga, caused by both natural–climatic and anthropogenic variations in the hydrological cycle.     

Probabilistic Regularities in Setup Variations in the Level of a Large Inland Water Body: Case Study of the Caspian Sea

The results of probabilistic analysis of data on setup level variations collected during long-term observations at all gages along the Caspian Sea coast are discussed. A procedure for evaluating low-probability sea level extremes is proposed. Estimates are given for the probabilities of outstanding setups in the Caspian Sea and the Sea of Azov.     

Regularities in sea level rise impact on the hydrological regime and morphological structure of river deltas

The problem of assessment of sea level rise impact on the hydrological regime and morphological structure of river deltas is discussed. Studies of the response of river deltas, which are among the most vulnerable natural objects, to the sea level rise has become urgent because of the global climate warming and the associated acceleration of the World Ocean level rise. Methods are described that can be used for the analysis, calculation, and prediction of sea level rise impact on submergence of deltas, propagation of backwater from the sea tides, surges, and salt seawater intrusion. Special emphasis is given to channel processes in delta branches, which accompany sea level rise, as well as to delta coastline erosion and flow redistribution among branches. In the course of research, due consideration was taken of the experience gained in studying the response of river deltas on the Caspian Sea coasts to the recent considerable level rise in this water body.