Floodplain vegetation and channel dynamics of a dam-impacted large lowland river: Satellite-based study of the Drava River, Croatia

Rivers are highly dynamic environments shaped by strong interactions between riparian vegetation and geomorphic processes, which are influenced by both climatic drivers and human activities. Understanding the functioning of river–floodplain systems is essential for river management, including flood protection and conservation of riparian zones. Satellite imagery provides an effective tool for analysing river and floodplain dynamics, as it offers high temporal resolution and spatial resolutions adequate for medium to large river systems.

The Drava River is one of the best-preserved large lowland rivers in the wider Pannonian region, largely due to its relatively wide riparian corridor. However, the river has experienced substantial human impacts, including channel straightening and meander cut-offs since the 19th century, as well as the construction of three hydropower plants and their associated reservoirs in the Croatian reaches during the 1970s and 1980s. These interventions have led to reduced sediment load, resulting in channel incision and narrowing. Although the Drava River is now protected as a natural area, detailed studies of vegetation–morphology interactions remain scarce. Existing research is largely based on spatially limited field surveys or historical maps with low temporal resolution.

In this study, we employ Landsat satellite data to investigate long-term vegetation and morphological dynamics within the Drava River channel and floodplain, using cloud-based data processing and statistical modelling. The objectives are to analyse changes in channel planform and vegetation coverage, and to examine phenological patterns of floodplain vegetation in relation to groundwater levels and climatic variables. We hypothesize that altered sediment and hydrological regimes following dam construction promote channel narrowing and vegetation expansion within the channel and floodplain. Additionally, we assess how these changes, together with climate variability, affect floodplain forest phenology along a downstream gradient from hydropower dams. Preliminary results derived from classifications using vegetation and water indices indicate a slight but statistically significant increase in vegetation cover. This trend is consistent with findings from other European rivers and is likely linked to reduced sediment flux and prolonged dry climatic periods. The study aims to provide insights into biogeomorphic processes relevant for river management and restoration of the Drava River floodplain.

This work was supported by the Croatian Science Foundation for the ALCAR project: “Assessment of the Long-term Climatic and Anthropogenic Effects on the Spatio-temporal Vegetated Land Surface Dynamics in Croatia using Earth Observation Data” (Grant No. HRZZ IP-2022-10- 5711).