Studied catchments are parts of Zagros belt which is part of the Arabia–Eurasia collision zone. Zagros is an example of a young and active orogenic belt and geomorphic, geologic and seismic evidences reveal that uplifting is migrating from northeast towards the southwest.
Among geomorphological evidence of active tectonics, drainage systems and their characteristics like drainage pattern, drainage anomaly, confluence angle of drainages and river direction play an important role in identifying active tectonics and their Spatial variations.
In this study, with the aim of evaluation of effect of tectonic on drainage system anomaly, four catchments, Patagh, Piran, Kamandan and Daretakht have been studied. Patagh and Piran are located in Zagros Folded Belt and two other catchments are located in Zagros Thrust Belt.
General characteristics of catchments like drainage divide and topography were extracted from topographic maps at a scale of 1:50000. Geological units and their areas were derived from 1:250000-scale geological maps.
Hierarchical anomaly index (?a), Bifurcation index (R), asymmetry factor (AF), transverse topographic symmetry factor (T) and drainage pattern are studied parameters in this research. For calculating mention factors, at first, 20 meter topographic contour lines of catchments have been digitized in the ILWIS (Integrated Land and Water Information System) software and thereby Digital Elevation Model (DEM) and drainage system ordering by Strahler’s method have been prepared. After ordering process, number of channels for all orders was determined.
Then, anomalous drainage paths and number of rivers in each anomalous path were achieved. For calculating Hierarchical anomaly index, at first, Ha (minimum number of first order channels which would be necessary to make the drainage network perfectly hierarchized), were calculated for catchments. ?a was calculated as the ratio of Ha divided by the real number of first-order segments of the network.
Bifurcation index (R) was calculated as the difference between the bifurcation ratio and the direct bifurcation ratio . of channels with a given order was computed by dividing the number of channels of a given Order by the number of channels of the immediately higher order. Bifurcation ratio of a catchment was achieved by the mean Rb of all stream orders. Direct bifurcation ratio was calculated as:
Where represents the number of channels of a given order that flow in channels of the next, higher order, and is the number of channels of the next higher order.
The asymmetry factor (AF) was defined as AF=100 (Ar/At), where Ar is the area of the catchment to the right (facing downstream) of the trunk stream, and At is the total area of the catchment. Transverse topographic symmetry factor was calculated as T=Da/Dd, where Da is the distance from the midline of the catchment to the main river, and Dd is the distance from the catchment midline to the catchment divide.
Results and Discussion
Results of this study shows that the values of hierarchical anomaly index (?a) in Piran and Patagh, in the zone of Folded Zagros, are higher than Kamandan and Daretakht catchments located in the zone of Thrust Zagros. The values of mentioned index in Patagh, Piran, Kamandan and daretakht catchments are 2.06, 1.65, 1.24 and 1.15 respectively. The highest value of Bifurcation index (1.01) is in Patagh and the lowest value (0.75) is in Kamandan catchment. The T values of Patagh, Piran, Kamandan and daretakht catchments are 48%, 39%, 33% and 21% respectively. The values of AF in Patagh, Piran, Kamandan and daretakht are 63, 59, 57 and 56 respectively, indicating that all catchments have been titled towards left.
Data analysis reveals that there is meaningful positive relation between ?a and AF so that correlation coefficient (R^2) of these two parameters is 96%. Correlation coefficient between ?a and T is 87%. Nevertheless, studying the relation between R and AF shows that there no positive relation between these two parameters and that their R^2 is 44%. Correlation coefficient between R and T is also weak (23%).
Studying the drainage pattern of studied catchments illustrates that older and more eroded catchments (Kamandan and Daretakht) in Thrust Zagros have dendritic drainage pattern. Drainage pattern in Patagh and Piran catchments (in younger parts of Folded Zagros) is trellis. However, parallel drainages have developed in steep limbs of anticlines whereas dendritic pattern have established in structural gentle slope.
Overall, this research shows that drainage anomaly in studied catchments is affected by tectonics and, that ?a has more performance than R in determining active tectonics of catchments.