The investigation and analysis of the morphotectonics of catchment area can be helpful in identification of the area’s active morphotectonics and it can also provide us with some significant and valuable information regarding the district's tectonic features and its tectonic activities. Rivers are of such indicators that show extreme reactions to tectonic activities and therefore, the results of investigating their changes and alterations can be a suitable guide for analyzing and determining the amount of tectonic activities of the study area.
As a river is shaped under active tectonic circumstances, some morphologic alterations would take place in such structures as channels and longitudinal profiles in reaction to tectonic alterations. Investigating the rivers' drainage system is so important especially in thrust systems since these structures show extreme reaction to vertical tectonics and folds. The river’s bedrocks undergo eroding on the ground and their vertical profile alterations are of the most important geomorphological components regarding the development of mountains views. In situations when the river is not capable of removing the gradient anomalies, Knick points would be made; and if it creates a vast area, it would be called Knick zone which would be found in the shape of a waterfall in flow-paths with flow.
The study area is located in Southern Alborz to Varamin plain and Qom's salt lake. The controlling factors of the area's tectonics include lithology, climate cycle, and structural factors (fold and fault). The northern part of the area (the Southern hill of Alborz) is completely folded and faulty and has made a totally active area tectonically. The highest part of study area is Damavand summit and its height is 5610 meters above sea level. The south area has some hills and mountains; however, they are totally scattered and plains (Varamin plain and the surrounding areas) have occupied most of the space of the area. This area has created a very tectonically active area since it is located in Alborz foothill, and there are so important faults such as Mosha's thrust fault, the thrust fault of north of Tehran, Taleghan fault, Emamzade Davood fault, and in southern areas, garmsar, eivanaki, hesarbon, and gharbilak faults are located.
The northern half of the area, which is affected by orogenic activities of Alborz, has shown totally a higher amount of activities comparing to the other parts of the area. In this article, the active tectonics of the south of central Alborz in catchment areas of Jajrood, Eivanaki, and Hablerood have been measured in ARC GIS 10.1 environment using river indexes including Hierarchical anomaly (Ha), Bifurcation index (Bi), Hypsometric integral and curve (Hy), Basin shape (Bs), Basin relative relief (Bh), and Drainage density index (ρ). To this end, the study area has been divided into 18 basins and most of the basins related to the northern and north-eastern areas which are located in the uplift zone have recorded high tectonic activities in most of the measured data. Moreover, the basins have been divided into five levels according to the rate of their tectonic activities; level 1 indicates the highest rate of activities and level 5 indicates the lowest.
After measuring all the indexes, the active tectonic index (lat) was measured for all the study area in order to come up with an overall conclusion. This index also, confirmed other data and observations as expected. A number of 6 catchments out of 18, all of which related to the northern and north eastern areas, revealed the highest rate of tectonic activities, and their activity index was 1. A number of 11 catchments revealed medium activity, and they were mostly related to the central and southern areas of the study area; and only 1 catchment revealed the lowest tectonic activity which was related to the most southern study area. Totally, according to the data collected from the three catchment areas which were investigated in this study, the highest rate of tectonic activity is recorded in Hablerood catchment area and the lowest rate is recorded in Eivanaki (Galooshoot) catchment area, and Jajrood catchment area has revealed a medium rate of tectonic activities. The results showed that in catchments affected by the main faults of the area such as Mosha, Emamzade Davood and Porkan faults in the northern area and Hesarbon, Eivanaki and Garmsar faults in the central and southern area, high tectonic activities were recorded which were also confirmed by field observations.