Analysis of changes in the channel planform and river bank changes during the period 2009-2024 (Jalair River , Markazi Province)

Introduction: Over the past few decades, the Jalair River has been subjected to varying degrees of morphological changes, resulting in diverse consequences. Accordingly, this study was conducted to investigate and analyze evolutionary trends, examine the resulting changes, and ultimately identify the relationship between these trends and their controlling factors. The analysis was carried out across three time periods (2009, 2014, 2024) for river management purposes, focusing on two reaches: the first reach spanning 3 kilometers (divided into 2 sections) and the second reach spanning 4 kilometers (divided into 3 sections).
Materials and Methods: The Jalair River basin is one of the watersheds in the Central Iran region, located in the Khazarazan district of Tafresh County. It is a sub-basin of the Qarachai River, which discharges into the Salt Lake (Namak Lake). The highest point of the basin is the Siah Kamar Mountain in the southeastern part, with an elevation of 3074 meters above sea level, while the lowest point is at the basin outlet, with an elevation of 1238 meters above sea level. The required data for this research were collected using hydrometric data from the Jalair station, Google Earth satellite imagery, field observations, and surveys. Subsequently, the data were analyzed using ArcGIS, Google Earth, and Excel software. Given the total length of the Jalair River (7 kilometers), the study area was divided into northern and southern sections. For a more detailed examination, the river course was further segmented into upstream and downstream reaches based on changes in pattern, slope, and channel position. To investigate changes in the channel planform, the right and left banks of each river reach were initially delineated for different years (2009, 2014, and 2024). These delineations were then overlaid to calculate the displaced area. Finally, riverbank changes were analyzed using the River Network Change Index (RNCI) and planform change analysis.
Results and Discussion: The results obtained using the River Network Change Index across the three studied time periods revealed that in the first period, all right banks of the reaches in this section were of the aggradational type, with Reach 2 experiencing the highest sedimentation. The left bank of Reach 1 was aggradational, while the left bank of Reach 2 was erosional. In the second period, all right banks of the reaches in this section were erosional, with Reach 1 experiencing the highest erosion. All left banks of the reaches were aggradational, with Reach 2 exhibiting the highest sedimentation. In the third period, all left and right banks in Reaches 1 and 2 were aggradational. The highest average rate of periodic change occurred in the second period on the left bank. Furthermore, the analysis of channel planform changes showed that the two adjacent reaches exhibited contrasting behaviors over the 15-year period: Reach 1 experienced an 11% reduction in area (narrowing), while Reach 2 underwent an 11% increase (widening). This disparity reveals a strong dependence of each reach's morphological response on local characteristics and the combined effect of extreme hydrological events.
Conclusion: The change analysis indicates a predominant long-term trend of aggradation. However, the occurrence of intermittent erosional phases (particularly on the right bank) and intense changes in specific reaches suggest the influence of extreme hydrological events and non-uniform morphological responses along the river. A persistent pattern of asymmetry between banks is also observed, with the right bank exhibiting a greater tendency toward erosional behavior. Furthermore, different reaches demonstrated varied responses, with Reach 2 being a focal point of intense change. The results of the river planform change study also reveal that the studied river is a dynamic and non-uniform system. At the long-term scale (15 years), the dominant trend is aggradation, but this trend was not uniform along the river, as two adjacent reaches exhibited opposing behaviors.