Water Framework Directive (WFD) guidance documents from 2019 provide a comprehensive review of methods and guidelines to improve the comparability of heavily modified water bodies (HMWBs) in the European Union. However, there is currently no common, single methodology for monitoring HMWBs and artificial water bodies (AWBs) or for determining maximum (MEP) and good ecological potential (GEP). This study presents the first assessment of hydromorphological conditions of HMWBs and AWBs in Croatia based on type-specific indicators. The typology of HMWBs and AWBs was based on distinct hydromorphological characteristics and modifications in order to be easily related to the uses of the water bodies. The classes of hydromorphological potential were graded from the theoretical MEP, which was determined by the score scale as the tolerated deviation from natural reference conditions, considering potential mitigation measures. The use of the water body and/or the effects on the wider environment were considered while determining MEP and choosing indicators included in monitoring and assessment. In the case of AWBs, the parameters dependent on natural reference conditions, which are non-existent, were omitted from the assessment. Only 27% of HMWBs and 2 out of 51 AWBs achieved good potential for all three hydromorphological elements. The most significant hydromorphological modifications include channelization, straightening, deepening and removal of riparian vegetation due to flood-protection management practices. In order to achieve the environmental objectives set out by the WFD, the Croatian water management system has to start implementing mitigation measures, especially related to natural flood management.

This study employs object‐based image analysis to investigate land cover dynamics and channel changes in the managed corridor of the Orljava River following anthropogenic vegetation removal and a flood event. By classifying RGB and near‐infrared (NIR) images from the decade 2011–2021, five land cover classes within the river corridor were mapped: water, bare soil, sparse vegetation, dense vegetation, and shadows. A digital surface model generated from the images was used to differentiate between bare river channel units (river sediments) and bare soil in the floodplain, as well as to identify high vegetation, while agricultural land was classified manually. The study identified two main phases of river corridor changes, driven by a significant flood event in 2014. In the period 2011–2014, which includes the flood event, a substantial increase in the areas covered by water and river sediments was observed. The most notable decrease was in the bare soil class (90%), which primarily covered riverbanks and adjacent areas in 2011 after vegetation removal. In addition, the formation of large in‐channel bars due to sediment accumulation and significant channel migration was recorded. In the second phase (2014–2021), lower discharges facilitated gradual channel recovery, characterized by channel narrowing and an increase in dense and high vegetation. This study demonstrates the high reliability of object‐based classification in mapping river corridor units using optical photogrammetric data. Furthermore, it highlights the deficiencies in inadequate river management practices that led to considerable bank erosion and a loss of agricultural land.

Although rivers are inherently dynamic systems that are susceptible to change, human impact on rivers in the last century is considered to have been so significant that it has caused an unprecedented intensity of geomorphological change in river channels and floodplains. As these changes often lead to deterioration of ecological conditions as well as increased flood risks, the approach to river management has changed over the past twenty years. There is an increasing emphasis on a holistic approach based on the understanding of river system processes, for which studies of geomorphological change in rivers represent a very important source of information. The aim of this review is to present the basic methods used in studies of geomorphological change in rivers, including the spatio-hierarchical delineation of the river system, data sources, and the most commonly analysed geomorphological characteristics and factors of change. The results of previous research are presented for the period of the last 150 years. The most important geomorphological changes include channel narrowing, incision, and reduction in the complexity of fluvial landforms and processes due to channelization and the construction of numerous barriers that disrupt the connectivity in water flow and sediment transport. Explaining the cumulative impacts and predicting future changes are the major research challenges. These challenges are related to the complexity of the river system, i.e. a large number of causal factors, connections, and interactions in the river system, and to the nonlinearity of the evolutionary trajectory of changes in rivers.

This study presents methodologically consistent length measurements of rivers with a total length of 40 kilometres or more in the Republic of Croatia. These measurements were performed by vectorising river channels from a topographic map at a scale of 1:25,000. Contemporary orthophoto maps were used as additional cartographic sources for verification. The approaches used to determine the starting and ending points of rivers, i.e. sources and mouths or confluences, have been explained and their spatial coordinates were listed. The results showed that in terms of total river length, the 21 rivers in this study differed from the available official data by less than 5%. Considerable differences were found for rivers where significant river engineering works had been carried out to channelize and divert flow, as well as for rivers in the Adriatic Basin where there have been methodological differences in determining the river mouth, and for large lowland rivers where changes in channel planform are common due to natural dynamics and engineering interventions. Moreover, this research revealed that the longest Croatian river, with its source, confluence, and both river banks situated entirely within the territory of Croatia, is the KaraSica River, with a total length of 148 km.

The development of remote sensing technologies and image classification methods has facilitated research on changes in river channels and floodplains by automating mapping of land cover and geomorphic units. In often highly heterogeneous river environments, object-based approaches proved sensible, since they are based on objects produced by image segmentation rather than on individual pixels.This study uses object-based image analysis to investigate land cover and river channel dynamics in the managed corridor of the Orljava River in the Pannonian basin (Croatia). In the last decade, the river has been affected by anthropogenic removal of riparian vegetation to increase channel capacity, which was followed by a big flood event. Five river corridor classes (water, bare soil, sparse vegetation, dense vegetation, and shadows) were classified based on RGB and near-infrared (NIR) aerial images in the period 2011-2021. A digital surface model generated from the images was used to separate bare river channel units (“river sediments”) from bare soil in the floodplain and to define high vegetation, while agricultural land was classified manually. The accuracy of the produced maps was between 85 and 93%, except for the year 2014 which lacked the NIR band. Based on classified river corridor units, changes in channel morphology were further analysed in GIS. The two main phases of river corridor changes were caused by the occurrence of a big flood in 2014. In 2011-2014, immediately after the flood, a significant increase in the area of water and river sediments was recorded, mostly at the expense of bare riverbanks and adjacent agricultural land. Large in-channel bars have formed due to sediment accumulation, as well as significant channel migration has been recorded. Contrarily, in 2014-2021 lower discharges allowed gradual channel narrowing and stabilisation, characterised by the spread and growth of vegetation in the river corridor.Observed changes in channel morphology and vegetation succession are natural processes related to actively meandering rivers. However, it has been shown that extensive bank erosion during the flood and subsequent land cover dynamics were primarily triggered by anthropogenic removal of riparian vegetation exemplifying how inadequate and isolated river management decisions may increase bank erosion and lead to a loss of agricultural land in floodplain areas.

Riverbank erosion is a natural process in meandering rivers that contributes to sediment supply and geomorphic diversity, yet it can threaten infrastructure and human activities within the floodplain. Recently, many studies have used high‐resolution remote sensing technologies to measure bank erosion, but they often focus on technical aspects or platform comparisons rather than analyzing the geomorphic and hydrological drivers of its spatial variability. This research aimed to investigate short‐term patterns and controlling factors of bank erosion at 10 meander bends along an actively eroding gravel‐bed river. Repeated field surveys involving UAV (unmanned aerial vehicle) data acquisition and GNSS (Global Navigation Satellite System) measurements were conducted between March 2022 and May 2024, covering periods of both significant floods and low‐flow conditions. Three key bank erosion metrics were calculated: bank retreat rate, area of eroded floodplain, and volume of eroded material. Riverbank lines were automatically vectorized using Laplacian edge detection filtering of digital surface models (DSMs) in GIS, and erosion volumes were quantified through temporal DSM differencing. The results indicated that erosion rates were positively correlated with meander sinuosity only during the high discharge period. Greater erosion was generally measured in meanders with grass‐covered banks compared to those stabilized by woody vegetation. This study contributes to understanding the short‐term dynamics of riverbank erosion at high spatial resolution, particularly under varying flow conditions. It also presents a practical workflow based on UAV data for rapid bankline delineation and erosion quantification, offering a valuable tool for river management.