بررسی تأثیر احداث سد مخزنی کرخه بر ریخت‌شناسی رودخانه با استفاده از شاخص‌های هیدرولوژیکی و تصاویر ماهواره‌ای

Background and objectives:
Construction of large dams alters riverine erosion and sedimentation patterns, exerting substantial impacts on channel geometry and fluvial morphodynamic stability. Meandering rivers are inherently dynamic systems that continuously evolve under the influence of erosion, deposition, and anthropogenic activities. Such processes can lead to meander migration, bank erosion, and an increased risk of flooding. Reservoir dams markedly restrict downstream water discharge and sediment transport, thereby modifying channel width, depth, and cross-sectional form. The Karkheh Basin is one of the most vital and strategic basins in West Asia and plays a key role in Iran’s food security. Following the impoundment of the Karkheh Dam in 2001, the mean instantaneous discharge downstream of the reservoir decreased by 78%. Studies indicate that after dam construction, reaches that were previously stable have become morphologically unstable. The river width has decreased from 273 m prior to dam construction to approximately 60 m, and lateral channel migration has been reported at about 34 m per year. The present study investigates the morphological changes of the 212-km Karkheh River from 1985 to 2024 and analyzes the controlling factors based on hydrological indices. Collectively, these objectives contribute to a clearer understanding of the impacts of the Karkheh Dam on river morphological adjustment and discharge variability.
Materials and methods
The core methodology of this research is centered on the temporal and spatial comparison of riverbed morphological changes. In the first stage, the location of the Karkheh River Basin and the study reach were delineated using 1:25,000 digital topographic maps and ArcGIS software. Relevant satellite imagery covering the study area was then collected. For more detailed analysis, the study reach was divided into three longitudinal sections. In the next step, geometric characteristics of meander bends—including bend radius, wavelength, central angle, valley length, and sinuosity—were extracted for each time interval using AutoCAD software. These data were subsequently analyzed using JMP statistical software. In this study, the impacts of the construction and operation of the Karkheh Dam on the hydrological regime and channel morphology downstream of the dam were assessed through discharge-based hydrological indices and meander geometry indicators. Considering that the Karkheh reservoir dam was constructed and impounded in 2001, monthly discharge data were divided into two phases for hydrological assessment: the pre-dam period (1985 and 1996) and the post-dam period (2014 and 2024). Furthermore, using Landsat and Sentinel satellite imagery, the geometric indices of the Karkheh River were analyzed to evaluate morphological changes over time.

Results
The results indicated that the monthly discharge time series at the Payeh-Pol hydrometric station underwent substantial changes from 1985 to 2024. Prior to dam construction, an increasing trend (+1.06) was observed, whereas after dam completion, a pronounced decreasing trend (–1.65) emerged, reflecting a significant reduction in flow volume. Analysis of the hydrological indices Q95, Q50, and Q5 confirmed considerable declines, particularly in low flows (a 59.7% decrease in Q5), which signifies heightened environmental stress and ecological vulnerability in downstream habitats. Findings also showed that the baseflow index increased by 58.4%, indicating a higher contribution of stable flows to the overall flow regime. Additionally, a 42.9% reduction in the duration of extreme flows, along with a 31.1% increase in the IHA index, revealed an intensification of flow regulation and disruption of the river’s natural hydrodynamics. In terms of channel geometry, satellite-image analysis and meander delineation within the GIS environment demonstrated that, following dam construction, the number of meander bends increased by 60%, 33%, and 29% in the upstream, middle, and downstream reaches, respectively. Furthermore, sinuosity and central angle indices across different periods showed a clear increasing trend in meander curvature in the post-dam era. Specifically, the proportion of highly sinuous bends in the upstream reach increased by 55%. Overall, the results suggest that reduced discharge and shear stress downstream of the dam have facilitated the formation of more numerous yet gentler bends. Thus, discharge regulation by the upstream reservoir—while effectively attenuating extreme floods—has shifted the river’s morphology toward increased meandering complexity and has altered the system’s morphodynamic parameters.
Conclusion
The 39-year monitoring results of the Karkheh River (1985–2024), based on Landsat and Sentinel-2 imagery, show that the construction of the Karkheh Dam in 2001 fundamentally altered the river’s hydrological regime and morphological behavior. Hydrological indices indicate that both low and high flows have been severely constrained. Moreover, the 58.4% increase in the Base Flow Index (BFI) reflects a more uniform discharge regime and a reduction in the river’s natural flow variability. These hydrological shifts have contributed to reduced shear stress, enhanced sediment deposition, and ultimately, significant morphological transformation downstream of the dam. Geomorphological analyses revealed a 29–60% increase in the number of meander bends across the longitudinal sections of the river after dam construction, suggesting the subdivision of large, pronounced bends into smaller and gentler ones. Although sinuosity experienced a slight decline in all three sections (approximately 5–10%), the number of bends categorized as “highly sinuous” increased by 55% in Section 1 and by approximately 25% in Sections 2 and 3, indicating intensified small-scale meandering and channel narrowing. In some reaches, channel width decreased by up to 78%, leading to lateral migration rates of about 34 meters per year. In summary, although the Karkheh Dam has reduced flow variability and flood hazards, it has simultaneously intensified lateral instability, increased the number of bends, narrowed the channel, and altered the meander pattern. Therefore, future management should incorporate controlled flood releases, sediment management strategies, annual satellite-based monitoring, and stabilization of vulnerable banks to prevent further instability and protect critical infrastructure.