Analysis of Equilibrium Response of Damghan Area Rivers to Tectonic and Erosion Processes Using SPL Model

Introduction
Landforms and their response to environmental changes are among the most interesting topics of geomorphologists. One of the landforms that is most affected by tectonic and erosion processes is the rivers. They respond to tectonic processes that increase the height of landscapes and erosion processes, trying to reduce the landforms’ height. This reaction can be well studied by analyzing the longitudinal profile of the rivers. One of the effective parameters in the study of tectonic and erosional status of regions is steepness and concavity, two parameters which could be examined in form of Stream Power Law (SPL). The function is related to the incision power of streams, giving a relation between slope and drainage area of the river in a logarithmic plot based on power regression, value extracts of the two parameters, and the channel’s steepness and concavity. In fact experimental studies by other researchers have shown that there is a direct correlation between steepness and concavity of rivers and tectonic – erosive processes in the regions.
It is generally accepted that steep landscapes are associated with areas of high uplift rate and active tectonic. River systems are well adopted to tectonic processes to provide useful information about the rate of uplift in the landforms. The steepness of rivers, which depends on channels’ declivity, is a fraction of uplift rate. Thus, it is expected that if steepness in the longitudinal profile of the river is low, the uplift rate is slight, and if it is high, the uplift rate becomes intense. Concavity index usually depends on bed material; however, erosion efficiency has direct connection with incision power law and its steepness. At the same time, weakness of bed materials, especially alluvial, can boost the rate of erosion efficiency in channels. This latter parameter is the volume of sediment, completely removed from the environment after erosion. A function of sedimentary flux, it can be directly related to the tectonic processes as well as bedrock characteristics. If tectonic processes lead to an increase in the landforms’ height, they are capable of increasing orographic precipitation in mountainous areas, leading to an increase in sedimentary flux, which in turn boosts erosion efficiency as well. The main purpose of this study is to analyze the effect of active tectonic and erosion on equilibrium profile of the main rivers of Damghan Mountains, based on the Stream Power Law. Both steepness and concavity parameters are affected by a set of lithological, geological, topographic, and erosion factors, all of which are effective for the location of rivers’ knick points and are able to provide useful information about the geological and erosion status of the area.
Material and Method
In order to investigate the power incision law, the DEM map with a resolution of 30m was utilized to extract the channels. When extracting the rivers, their flow direction was calculated via D8 algorithm method, which measures the flow path of each pixel, falling on the lower pixel with a lower slope, thus determining the flow directions. In this regard, we first need to create a DEM map with the least inconsistency. This method focuses on extracting central flows in valleys and reducing parallel flows. Hence after extracting the channels, their slope-area logarithmic diagram got plotted. The regression line considered for the logarithmic plot was the power regression, the relation of the river incision power. In this regression, the slope of the regression line is concavity and the intercepting line, the steepness. To obtain information about lithological features of the area that are effective for analysis of both concavity and steepness parameters, the geological map of Damghan and Shahrud was used. The study area is part of the mountainous structure of Eastern Alborz, having several active faults. North Damghan Mountains are located on the southern side of eastern Alborz between 36^。14^'0.3^" to 36^。18^' 82^" and 55^。00^' 26^" to 53^。59^' 56^" in north of Iran plateau. There are different outcrops of lithostratigraphic formations from Precambrian to Quaternary in this area. Geologically speaking, the study area is composed of a set of over thrust blocks and nappes. The thrust faults and nappes within piggy back style have pushed eastern Alborz stratigraphy sequences on each other. The folds in the region have a strong connection to thrust structures and nappes. These folds are of different types and sizes, but most of them are inclined and recumbent because of the widespread compressive component in eastern Alborz.
Result and Discussion
The three main rivers of the region, Cheshmeh Ali, Astaneh, and Tepal were studied. All three rivers flow on the colluvium bed in the upstream and alluvial bed in the downstream. Furthermore, all three are affected by faults in some areas. Some, such as Cheshmeh Ali River in the southern part, flows into a fault valley, where the activity of faults along the rivers, both in the resistance and alluvial parts, has uplifted the rivers. These effects can be seen in the high values of steepness index and low values of concavity index. The increase in the stream incision is seen in both the upper and lower section of the rivers due to the activity of faults in the region. However, the steepness is higher in the upstream, which is made of colluvium sediments. While downstream, due to the weakness of alluvial sediments the rate of erosion efficiency is higher. Therefore the change in the rate of steepness, concavity, and erosion efficiency, in addition to active tectonic, is strongly affected by the channels’ bedrock. Each rivers that is faultier also has higher values of the steepness index. Cheshmeh Ali River, part of which is located completely in the faulted valley, has the highest rate of steepness, compared to other rivers. Astaneh River is affected by Astaneh fault in several parts, and the fault has uplifted the river by cutting off the Quaternary sediments. The high values of steepness parameter in this river confirm existence of active tectonic. In its upper part, Tepal River shows high values of steepness, but downstream, where the river flows on agricultural lands, the erosion efficiency rate increases and, in contrast, the steepness rate decreases. This is due to human activities, which exceed the rate of erosion from that of tectonic processes. Therefore, human activities are able to transform the relations between internal and external processes that are effective in changing landforms.
Conclusion
Results show that reaching the equilibrium profile in each river depends on a set of factors, such as erosion, tectonic, and lithology. Presence of a fault in the channel path increases the height and slope of the river channel, leading to further erosion in response to this.  Tectonic processes boost the incision capacity of rivers as a result of increasing their channels’ slopes, which in turn leads to production of more sediments in the river. Of course, similar to Tepal River, we must consider the role of human activities in increasing the rate of erosion efficiency.