Probabilistic Modeling in Investigating the Factors Affecting KnickpointsCase Study: Zagros Mountain Belt, Ghaleh Shahrokh Basin

Rivers react to subsidence at their baseline by cutting and digging topographic features. The development of an upstream incision is often accompanied by a steep fracture called a river break (Loget & Van Den Driessche, 2009). The presence of river breaks in a geographical landscape is an indication of a steady-state in river systems. Therefore, the presence of knickpoints shows the system instability. The study of knickpoints can be used in the field of studies related to the evolution of valleys, identification of tectonic active areas and rock outcrops, river surface changes, erosion and sedimentation, and geomorphological changes in river systems. The basin studied in this study is located in the Qaleh Shahrokh-Chelgard area in the northeastern part of Chaharmahal and Bakhtiari province, Iran. The reason for selecting this basin is the extensive activities of the Zagros fault along the northwest-southeast and the existence of a hydrographic network affected by the trend of faults and the potential for knickpoints.  

In this study, the locations of knickpoints were detected from the Radiometrically Terrain-Corrected (RTC) model which is extracted from the active microwave sensor ALOS PALSAR with a spatial resolution of 12 meters (Logan et al., 2014) as input data to the MATLAB executive toolbox called Tec DEM. Tec DEM is an executable toolbox in MATLAB software and uses a Digital Elevation Model (DEM) as input for morphotectonics in the basin. Tec DEM tool can be used in a variety of fields in the analysis of surface anomalies, drainage network and surface dynamics of basins, production of base maps, incisions (local roughness), vertical dissection and drainage density of basins and sub-basins, determination of turning points or knickpoints, hypsometric analysis and slope and concavity index of canal profiles (Shahzad & Gloaguen, 2011). The determination of knickpoints according to the shape of the longitudinal profile of the river is done semi-automatically. In this study, these points in the study areas were investigated according to field observations. In this study, geological variables and geomorphic variables related to knickpoints were used to identify the knickpoints. Information layers including geology, distance from the fault, distance from the boundary of geological formations, surface roughness index, fractal dimension, base surfaces, local roughness, and the vertical dissection as predictor variables and the layers of knickpoints as the prediction variables were used for modeling. For geological and tectonic studies of the region, geological maps of 100,000 sheets of Chadegan and Fereydunshahr and 250,000 sheets of Shahrekord were used. A total of 8 raster layers were used to analyze and predict the possibility of the presence of a knickpoint in the study area. Since 8 layers have different units and are not suitable as direct input for logistic regression, the input parameters were normalized in the range of 0 to 1. Nominal layers, such as geological data, became sequential variables between 0 and 1. All of these layers were then re-sampled as a network format with a cell size of 195*195 m using the nearest neighbor method, to allow all layers to be combined. Then, a matrix of square cell structure was prepared for the study area. It consisted of a matrix of 273 rows and 273 columns representing a total of 39,650 cells. Of these, 74 cells were identified as knickpoint points. These areas were identified with code 1 (presence of knickpoint) and the rest of the cells that did not have knickpoints were recorded with code 0 (absence of knickpoint).  

The probabilistic relationship of the presence of a knickpoint as one of the important results of the research was obtained by the logistic regression method. This relationship predicted the probability of the presence of knickpoints based on geological and geomorphic variables. The probability map of the knickpoints in the study area was obtained based on the statistical relationship. According to the results, there is a possibility of river knickpoints in the southwestern regions and parts of the northeastern Basin. The results of the probability ratio test to determine the statistical significance of each of the independent variables in the proposed model showed that the geology and the distance from the boundaries of the geological formations in the model were significant. The results of the Yuden index for the training dataset, validation, and test data were equal to 0.72, 0.76, and 0.66, respectively, which indicated the accurate information on the probability status of knickpoint points, especially for the test data of the model. The results of the Kappa agreement coefficient for training, validation, and test data were also equal to 0.62, 0.73, and 0.60, respectively, which indicated the agreement of both methods with the observed values ​​of knickpoints.  

The results of this study showed that at the boundary of lithology, because of the presence of joints and cracks due to differences in the type of rocks, the probability of the presence of river break was more than other parts of the region. Although the presence of some relatively high slope knickpoints indicated active tectonics in that area, in the present study, the effect of the fault system or active tectonics in the formation of knickpoints was not statistically significant. Particularly, the reduction of local roughness index and baselines was associated with less tectonic activity, but in this study, the appearance of knickpoints has been associated with a decrease in these two factors.