Introduction

River pattern changes are one of the most important river engineering issues that affect construction activities and structures on the banks of rivers. It is important to study the morphological changes of river channels in order to find appropriate control strategies to solve the dynamic problems of these areas. In recent years, changes in river systems have increased the environmental damage caused by river processes in many countries and have attracted the attention and sensitivity of managers, although change is an integral part of all river systems. Studying the patterns of rivers is essential to understand the current conditions and the potential for their possible changes in the future, and only in this way can their natural response to natural or human changes be detected, the rate of displacement, changes in their dimensions and patterns.

Therefore, the aim of this research was to investigate the morphological changes and dynamics of the channel over the last 60 years and the relationship between the geological characteristics of the region and changes in the planform of the Cheshmeh Ali Damghan River.



Methodology

Astana watershed is located in the northern part of Semnan province. The study area in this study is the Cheshme Ali River, which is formed by the confluence of the Astaneh and Damghanroud rivers and the outflow from the Cheshme Ali spring at the village of Ahwano. This river has a permanent flow and has a general flow direction from northeast to southwest until it reaches the Damghan reservoir dam at the end of the study reach. The purpose of analyzing the course of the river in the studied area from Ali Spring to Shahid Shahcheraghi Dam, 10 km long, is to separate 7 separate reachs, and further, the changes of the channel planform and its changes based on the area of the channel in different years, as well as the dynamics of the channel in different time intervals for each of the reachs of the river was calculated.

Results and Discussion

Morphological Changes: The analysis reveals significant changes in channel morphology across different river reachs. Reach 1: There was an increase in channel width and length due to increased curvature until 2003, followed by a notable reduction in the area of the channel platform from 31,649 km² in 1961 to 17,504 km² in 2013. This trend continued with further reductions observed in 2023. Reach2: A similar pattern was noted, with a drastic decrease in channel area from 29,478 km² in 2003 to 4,272 km². The data suggest that reduced curvature and channel width contributed to these changes. Reach 3: Initially, there was an increase in channel area until 2003, followed by a significant decrease by 2013. However, some recovery was noted by 2023. Reachs 4 to 7: These segments exhibited varying degrees of morphological change, generally characterized by reductions in channel area and width over time.

Hydrological Dynamics: The study also examined channel dynamics through the calculation of channel mobility across different timeframes. The highest mobility was observed between 2003 and 2013, particularly in Segment 7, which indicated a mobility value of 0.65. This suggests that this segment experienced significant morphological adjustments during this period. In contrast, Segment 1 demonstrated the least mobility (0.27), indicating more stable conditions compared to other segments.

Geological Influences: The geological composition of the study area plays a crucial role in shaping the river's morphology. Dominant geological formations include Quaternary deposits and Jurassic rocks across various segments. The lithological characteristics influence erosion rates and sediment transport, which are critical for understanding river dynamics. The variability in geological formations correlates with observed morphological changes, suggesting that geological stability or instability can significantly affect river behavior over time.



Conclusion

The findings highlight a clear trend of morphological change and hydrological dynamics within the studied river system over the past six decades. The reduction in channel area across multiple reachs indicates potential environmental stressors affecting river stability, such as anthropogenic impacts or Topographical features dominate the changes in slope and channel position due to channel restrictions. Significant reductions in channel area and width indicate a trend towards more stable yet potentially less diverse river habitats. Increased mobility and activity channel in certain reachs suggests active geomorphic processes that may be driven by external factors such as land use changes or climate impacts. Geological factors play a vital role in influencing both the physical characteristics and the dynamic behavior of the river. Also, changes in the channel planform and the surface occupied by the river have been different in different reachs and at different time intervals.