This paper investigates the impact of land-use changes on the channel morphology of the Qara-Su River and its consequences. The extraction of land-use maps was carried out using Landsat (2005) and Sentinel-2 (2024) satellite imagery. Subsequently, the effects of these changes on the river width, channel bed area, and the migration of the river’s centerline were examined. The results indicated that over the past 19 years, agricultural land-use within the river’s floodplain has expanded (e.g., 116 hectares in the first reach, 252 hectares in the second reach, and 203 hectares in the third reach). This expansion reflects the encroachment of agricultural activities into the riparian zone and even the active channel of the Qara-Su River. Furthermore, the analysis of river width changes at 38 selected cross-sections along the Qara-Su River clearly revealed a reduction in river width over the past 19 years. Measurements conducted on these 38 cross-sections indicated that the most frequent river width of the Qara-Su River was approximately 13 meters in 2005, while in 2024, the most frequent width decreased to around 4 meters. These changes thus indicate a significant narrowing of the river channel, which consequently reduces its capacity to discharge surface flows. Therefore, the risk of destructive flooding and the vulnerability of agricultural lands along the river's riparian zone are expected to increase significantly in the future. Consequently, attention to land-use changes in the Qara-Su River floodplain and the development of a comprehensive plan for managing human activities within the riparian areas and the river bed should be considered a primary priority for the relevant authorities.
Introduction
Rivers, as fundamental components of the hydrological cycle, play a crucial role in sustaining ecosystems and flood control. They are vital elements of terrestrial water systems and serve as the primary conveyors of surface water resources. Globally, areas near rivers are preferred for agricultural development and population settlement, and human civilization prospers along rivers (Wang et al., 2022). Land-use changes, such as urbanization and agricultural expansion, can significantly alter the morphological dynamics and hydrological behavior of rivers. River morphology plays a crucial role in channel stability, flood control, and ecological conservation, and changes in river morphology can have multifaceted impacts on the surrounding environment and society. These impacts include changes to river and adjacent ecosystems, shifts in the distribution and availability of water resources, and hydrological hazards in nearby and downstream areas (Qiang et al., 2007; Chakraborty and Datta, 2013; Ospide et al., 2017). The application of advanced technologies, such as remote sensing, GIS, and hydromorphological modelling, is essential for the analysis and management of river systems. The Qara-Su River in Ardabil Province has experienced significant physical changes due to the expansion of agricultural lands within its bed and riparian zones. Investigating the impacts of these land-use changes on river morphology is essential for the conservation of natural resources and the implementation of sustainable management strategies.
Material and Methods
In this study, multiple data sources were utilized to analyze land-use changes and the morphology of the Qara-Su River. Topographic and geological maps, ALOS-PALSAR elevation data, Sentinel-2, Landsat, and IRS satellite imagery were employed to generate a Digital Elevation Model (DEM), extract stream networks, and assess land-use changes. Hydrological data, including daily discharge and sediment load, were obtained from the Ardabil Regional Water Company and field surveys and were incorporated into the analysis. Data processing and analysis were carried out using software tools such as ArcGIS, ENVI, Sas Planet, Google Earth, HEC-RAS, and related plugins. Field data were controlled and validated with GPS. For land-use classification, a supervised classification method using the maximum likelihood algorithm was applied, with classification performed for both images based on 128 training samples. To quantify river width changes, the Lateral Mobility Index can be applied. This index is calculated by measuring changes in the outermost bankline of the channel (Richard, 2005):
In this context, a value close to 100% indicates the maximum lateral changes of the river, while a low value of this index represents minimal lateral changes. For calculating lateral migration rates was offered by Shields et al. It uses the following equation (Giardino and Lee 2011; Shields et al. 2000):
(2)
  Rm = (A / L) / y

Where Rm refers to lateral migration rate, A is the area between two centerlines of the channel, L is the length of centerline at time t1, and y is the number of years. The lateral stability of the channel is determined by the changes in the river's active channel without vegetation during the study period. In other words, the lateral stability index is calculated by dividing the unchanged active channel area by the previous active channel area:
When the output of this index approaches one, it indicates that the channel has remained in place, while values closer to zero reflect changes in the channel over the study period (Richard et al., 2005a, b).
Results and Discussion
In this paper, land-use changes and the morphology of the Qara-Su River over the past 19 years were investigated. The results indicated that between 2005 and 2024, agricultural activities within the floodplain of the river, particularly in its bed and riparian zone, have significantly increased. In certain reaches, such as the first reach, 116 hectares, and in the second reach, 251 hectares of agricultural land were expanded. These changes have led to the narrowing of the river channel and a reduction in the river’s flood conveyance capacity. For example, in 2024, the average width of the river channel in certain reaches decreased from 13 meters in 2005 to 3 meters. The construction of the Sabalan Dam has not only reduced the river's discharge and induced morphological changes downstream but also resulted in the stabilization of sediment bars and a further reduction in the channel width. These changes have led to a reduction in the river's lateral dynamics and an increased risk of major floods with return periods exceeding 10 years. Consequently, the diminished conveyance capacity and the potential for overbank flow into the floodplain pose significant threats to agricultural lands and downstream areas. The results showed that over the past 19 years, the river migration rate in different intervals ranged from 0.29 to 0.52 m/year, with an overall average of 0.44 m/year. The river area decreased from 80.04 ha in 2005 to 26.02 ha in 2024, indicating channel narrowing and a reduction in river width. Additionally, the lateral stability index for the intervals was calculated to range between 0.16 and 0.32, with an overall average of 0.22 for the river. The lateral change index ranged from 71% to 85% across different reaches, indicating significant lateral changes of the river. Even in areas with geological control, high values of the index correspond with a reduction in river width.
Conclusion
The present study investigates land-use changes in the floodplain of the Qara-Su River over the period from 2005 to 2024. The results indicate that these changes have occurred rapidly, leading to the narrowing of the river channel and a reduction in its water conveyance capacity. Satellite image analysis, with classification accuracies of 93% for 2024 and 91% for 2005, reveals an expansion of agricultural areas within the river’s riparian zone and active channel, which has contributed to the stabilization of vegetation along the riverbanks. The construction of the Sabalan Dam upstream has led to reduced river discharge and an increased risk of severe floods with return periods exceeding 10 years. These changes have increased the vulnerability of the floodplain to flooding and reduced its flow conveyance capacity. Therefore, it is essential to closely monitor land-use changes within the floodplain of the Qara-Su River and to develop a comprehensive plan for managing human activities in its riparian zone, which should be considered a primary priority. For effective river management, it is recommended that policies such as annual land-use monitoring and the enforcement of agricultural restrictions along the riverbanks be implemented.