Investigating the Geomorphic Sensitivity of Taleghan River Emphasizing Riparian Vegetation

River sensitivity is affected by geomorphic forms and adjusting river ability (Fryzer, 20017). Taking into account the difference adjusting capability, the behavioral sensitivity of a river can be observed over the entire river. However, there is the major various change in a behavioral regime of a river, due to episodic turbulence or disturbances events (Philips, 2009). River behavior monitoring over time shows variable behavioral sensitivity, however, it can be dynamically evaluated, so that it causes more sensitivity in some rivers and flexibility in others. This study aimed to investigate the river sensitivity of the Taleghan River emphasizing the effect of riparian vegetation.

The research tools were aril photographs and satellite images for the study period (table 1). Taleghan River was classified into 3 sections upstream, midstream, and downstream with average elevations of 1936, 1875, and 1793m, respectively, and a minimum slope of 0.009 to a maximum of 0.013. Channel stability was calculated by analysis of channel planform indices, including BI, CA, and CW. the sustainable patches with approximately sustainable area and location were determined using overlaying the aerial photographs and satellite images. The vegetation patches with an area of more than 500m were considered and the vegetation patches lower than 500m with a detachment of vegetation cover. The riparian vegetation degradation was observed using field observation over the study period.

Based on the findings, the SI index was approximately similar for the entire river and it is in the high level of sensitivity; however, the SI index was increased over 1370-13855. So, the distribution of the change over 2006-2021 is lower than 1991-2006, although the change level in 2021 is more and the change distribution is lower. It means that the change processes in all reaches are approximately the same. The relationship between the CW and CA with the BI index was explained using an X-Y plot (figure 4). The findings showed a significant correlation between the BI and CA with CA. River geomorphic sensitivity and the change level decreased and increased, respectively in 2021 than the past period (table 4). 82 sustainable vegetation patches were detected on the left and right banks of the Taleghan River during the study period. The area of the vegetation patches was 19.51%, 32.92%, 31.70%, 13.41%, and 2.46% in sub-reaches 1, 2, 3, 4, and 5, respectively. The degraded patches were in the sub-reach 1 and most of them were located near the river banks because of the establishment of vegetation patches near the river banks (fluvial traces), river undercutting due to the high hydraulic tension, and decreased bed weight and erosion by water flow. In other sub reaches changes in the turbulence intensity profile caused the distance of maximum levels from river banks.

In this study, BI, CW, CA, and SI indices in the 1st sub-reach with sinusoidal-straight planform during the studied period are lower compared to other intervals, and the amount of sedimentation is higher compared to erosion. The geomorphic sensitivity has decreased significantly in 2021 compared to the previous period and the area of the sustainable patches with a length of 3603.18 meters was 27.89645 square meters. Therefore, the least changes in geomorphology were observed in sub-reach 1, while the area of the sustainable patches in this sub-reach is more compared to other sub-reaches. It was observed that the vegetation patches were established in the sub reach 1 and part of the sub reach 2 on the immediate side of the river, and it has played a significant role in reducing erosion, adhesion of soil particles, and preventing the separation of soil particles, and hence increasing cohesion against erosion. However, the level of vegetation degradation in sub-reach 1 is significant due to the increase of hydraulic stress gradient compared to other sub-reaches. The ascending order of geomorphic sensitivity was observed in sub-reaches 1, 5, 4, 2, and 3, respectively in 2021. Whereas, the ascending order of geomorphic sensitivity was observed in sub-reaches 4, 1, 2, 5, and 3, respectively in 2006. Meanwhile, sub-reach 1 had a balanced trend and on the other hand, geomorphic sensitivity in sub-reach 3 was more than in other sub-reaches in the studied period (Table 4). Our observations have shown that in the vegetation patches, a combination of woody vegetation can be introduced as ecosystem engineers; because they have caused the increase of sedimentation and the creation of sedimentary tails, and the formation of landforms. The engineering effect of vegetation typologies along the Taleghan River has mainly occurred in places with a suitable physical location for sedimentation related to flow characteristics. River engineer species can develop different response characteristics to hydrogeomorphic constraints, therefore making possible their establishment in an unsustainable and disturbed geomorphic environment.