عنوان مقاله [English]
Rivers are active phenomena and are one of the most important factors affecting the geomorphological processes of the earth and the erosion cycle whose ecosystems are completely dynamic and their boundaries and their morphological characteristics change over time and continuously. The bed and characteristics of river morphology according to time and under the influence of factors such as flood, tectonics, dam construction, climate change, land use and human intervention is changing. River morphology Conceptually, the science of knowing river systems in terms of geometric shape, river bed properties, longitudinal profile and study of river channel variations as a natural process in alluvial rivers. By studying the morphology of rivers, it is possible to understand the current conditions and the potential for possible changes, such as spatial and temporal changes of the bed and its morphology, in the future.
The Silakhor River in the northeast of Lorestan province, in the plain of the same name, is one of the most active tectonic areas in Iran. The river is affected by the active tectonic, active fans and human activities it has a change of bed. Thus, the purpose of this study was to identify the morphological changes of the Silakhor River in a 20-year period (1995-2005) section at a length of 61 km, estimating the amount of erosion, sediment, and unchanged river boundary in the three studied periods and such identification is the process of changing the shape and pattern of the river studied.
The study of temporal-spatial variations of the Silakhor River has been carried out in four stages. At first, the required tools such as satellite imagery, geological maps 1:100000, topographic maps 1: 25000 and GPS were prepared for use in various stages of the research. In the second stage, satellite images (Landsat 8) were prepared for 1995, 2005 and 2015, and the river route was extracted in 3 periods in ENVI software environment. In the third step, using the union tool in the Arc GIS software environment and the extracted paths in the 3 periods studied, The erosion, sedimentation and unchanged areas were determined in the three periods 1995-2005, 2015-2005, and 1995-95. In the fourth stage, after specifying the points of the river change and estimating the area of erosion, increase (sedimentation) and unchanged area in the three studied periods, the causes of morphological changes in the Silakhor River were investigated.
Results and discussion
The study of the spatial variations of the river extracted route in the three periods of 1995, 2005 and 2015, and their overlap, indicate that the river route and bed have changed during each of the three periods. So that in some areas of the river bed, has risen or decreased and in some places remained unchanged. But due to the nature of the meandring of the river in the Silakhor plain, the major changes were in sections 2 and 3 of the river.
Estimated changes in the Silakhor River indicate that the river has destructive and erosion power and the river canal has been moving in every three periods. But differences in displacements at any point in any period of time, as compared to other periods of time, are observed in the form of erosion, sedimentation or unchanged. But in general, in the first, second and third sections in the three periods studied, the right bank of the river was mainly erosion and the left bank was increasing (sedimentation). So that the maximum amount of erosion during the period 2005- 1995, the maximum increase (sedimentation) over the period 1995-2005 and 2015-2005, and the maximum unchanged area for the period 2005-2015.
The factors influencing the spatial and temporal variations of the Silakhor River are investigated in three Section, the effects of geological structures, the effects of alluvial fans and human activities. The results showed that in the northern part of the region, the river is more along the path of the fault shoots with the northwest-southeast trends along with the Dorud fault and some branches of the fault are almost perpendicular to the fault. The presence of such a factor has led to a change in the overall route of the river in the areas of fault and river confrontation and the river flows in a different direction than before and follows the path of the fault. The presence of large alluvial fans around the Silakhor River indicates that the large river curves follow the alluvial fans of its path. Also, the occupation of the land around the river, its conversion to fields and the constrict of the river bed and the removal of sand from the river bed are considered as the most important human factors of the river's morphological changes.
The results of the overlapping of the river course in three periods showed that in some areas of the river bed boundaries increased or decreased and in some places remained unchanged. But, due to the nature of the meandring River in the Silakhor plain, major changes were made in sections 2 and 3 of the river. Also, river time variations show that in the first, second and third sections of the three periods studied, the right bank of the river was mainly erosion and the left bank was increasing (sedimentation). Estimation of erosion, sedimentation and unchanged river boundaries also showed that the highest amount of erosion during the period 2005- 1995, the highest increase (sediment) was in 1995-2005 and 2015- 2015 and the largest unchanged area in 2005-2015.
Keyword: spatial - temporal changes, morphology, Silakhor River.