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The terrestrial floodplain ecosystems of the Lower Don are still poorly studied, although almost the entire lower reaches of the Don are regulated by reservoirs and low-head hydroelectric systems, which significantly change ecosystems and landscapes. The research was carried out according to our own methodology. Field studies were carried out on five model ecological profiles with a detailed analysis of vegetation and the position of surface groundwater at model points. Desk studies included analysis of statistical hydrological materials and satellite images from Landsat 2, 3, and 5 and Sentinel-2, as well as the determination of the state of the ecosystems and landscapes using a two-stage methodology for assessing disturbances in terrestrial ecosystems and landscapes as a result of changes in the water level in the study area. It has been revealed that the main factor influencing the change in the biotic components of ecosystems in the Lower Don reaches is the reduction in the amplitude of fluctuations in water discharges (and levels) in the annual cycle as a result of the river control and climate changes. The decrease in the frequency of flooding of the floodplain and delta areas by almost 4 times along with a 3.5 times decrease in spring flood discharges and a 2.2 to 2.3 times increase in low-water discharges (both winter and summer–autumn flows) has led not only to a rise of the Don channel level by more than 2.5 m, but also to a rise and stabilization of the groundwater level (GWL) in the floodplain and delta of the Lower Don, which has involved a succession of communities of the upper and middle floodplains by nontypical invasive species. On the upper floodplain, native oak and elm forests have been almost completely replaced by green ash (Fraxinus pennsylvanica) and boxelder maple (Acer negundo) forests. Typical willow (Salix alba) forests on the middle floodplain are replaced by indigobush (Amorpha fruticosa) forests in the near-channel floodplain and oleaster (Elaeagnus angustifolia) forests in the central parts of the floodplain. The forb meadows of the middle floodplain are replaced by almost monodominant reedgrass (Calamagrostis epigeios) meadows with thorny weeds of thistle (Cirsium) and cocklebur (Xanthium) species. Newly formed nontypical floodplain communities are better resistant to a year-round steadily high GWL. Assessment of the disturbance of the water level in the ecosystems and landscapes on a 5-point scale according to our own methodology has shown that the least disturbed (two of the five possible points) areas are the areas that are most distant from the hydraulic structures, namely, areas in the new and old Don deltas. The other profiles (3–5), located in the impact zone of the hydraulic structures, have a moderate degree of disturbance (three points), which currently requires only local hydraulic and forest engineering drainage measures, rather than changes in the hydrological regime of the river