[11; 15]), heterocyclic aromatic hydrocarbons (HAHs), aromatic amines, dioxins and furans [11], and different petro-chemical products [16] plastics and microplastics [17] with their different sorption and desorption capacities are wide spread both in industrial and mining areas [12] and in and around large urban concentrations. Concentrations, spatial distributions along and across the rivers, migration behavior of these pollutants during flood and within floodplain soil profiles were analyzed and assessed by these and many other authors. Due to the fact that most of the big rivers cross state borders, transporting their pollutant load from the upper section of the catchment to the lower section or to the sea, systemic studies of the transboundary rivers water resources, their suspended matter and their fluvial sediments is an extremely important task, also for Kazakhstan [18]. Figure 3: Water management basins of Kazakhstan (source: Institute of Geography and Water Security, Almaty, Kazakhstan) [8]. One of the largest and most important water basins of Kazakhstan, especially for Northwest Kazakhstan, is the Ural-Caspian basin, where not only the Ural River is a transboundary water body, but also the Caspian Sea is shared by five Caspian states. The Ural River (Zhaiyk) is the third longest river in Europe after Volga and Danube (total length--2428 km, including 1084 km on the territory of the Republic of Kazakhstan) with a basin area of ~380 thousand km\({}^{2}\). The upper basin is located on the territory of the Russian Federation (RF), and the lower part is in Aktobe, West Kazakhstan and Atyrau regions of Kazakhstan (Figure 4) [19]. The river marks the border between Europe and Asia. Another feature is that the main spawning of sturgeon reo, which are endemic species, can be found in the middle part of the Ural River. Within international cooperation on shared water courses, a basin approach of integrated watercourse management recognized as a key solution to many problems, which allows taking into account the ecological state of the natural ecosystem, its stability and self-cleaning ability. The problem of fair and equitable use of shared water courses with Russia is one of the important issues for the Republic of Kazakhstan. The Ural-Caspian water management basin includes the Ural River basin, the Kamysh-Samara lakes basin (the Karaozen and Saryozen rivers), and the Volga basin (the Kigash and Shora rivers). In recent decades substantial changes in water resources, mainly water level drop down due to anthropogenic flow reductions, have been observed, which poses a serious threat to the sustainable development of the natural and economic system of Western Kazakhstan. According to long-term data, since 1971, there has been a systematic and phased reduction of the average annual flow of the Ural River with a slight increase in 1990-1994. The value of the annual flow of the river compared to the long--term average [20], equal to 12.0 km\({}^{3}\), decreased: by 1995--to an average of 10.0 km\({}^{3}\), or 16.7%, and by 2016--to an average of 7.47 km\({}^{3}\), or 37.8% [21] (Figure 5). Figure 4: Transboundary basin of the Ural River. Reservoirs: 1—Verkhneuralskoye; 2—Magnitogorsk; 3—Irikkinskoye; 4—Sakmarskoye; 5—Aktobe [19]. In a study by [PERSON] et al. [19], there are currently 18 large and medium-sized reservoirs with a total volume of more than 5.5 million m\({}^{5}\) in the Ural River basin on the territory of the Russian Federation. The largest among them (c.f. Figure 4) are the upper Ural (600 million m\({}^{3}\)), Magnitogorsk (190 million m\({}^{3}\)) and Iriklinski (3260 million m\({}^{3}\)) reservoirs. These reservoirs store the runoff of the upper and middle sections of the Ural River, which provide the needs of the population and all economy sectors. Besides the reduction of the transboundary inflow, one of the threats to hydroecological security remains the anthropogenic impact on water resources of the Ural River.