The Causes of Morphological and Sedimentary Changes of Jajrood River Upstream of Latyan Dam

Abstract

Studying the morphology of rivers leads to a better prediction of the rivers' behavior. Two categories of natural and human factors affect the behavior of rivers. Natural factors such as floods, soil erosion, landslides and human factors such as construction and installation of facilities, change of land use along the river and sand removal from the riverbed have an impact on the intensification of river morphological changes. In this research, it has been investigated the morphological changes of Jajrood river and its causes upstream of the dam with an approximate length of 140 km. For ease of work, the river is divided into four zones: A) mountainous and high area, B) split zone under the influence of Mosha -Fasham fault, C) transition zone between the riffle and the mountain, D) the river riffle and Latyan dam. In general, Jajrood river is morphologically steep (average slope of 4%), mountainous, braided and gravel. The torsion factor of the river is 1.06. Jajrood River is a straight river upstream, and braided and gravel downstream that particle size and sorting changes from upstream to downstream does not follow a specific trend. Tectonic activity in the Mosha Fasham fault causes the large particles to enter the river environment as the debris streams. Some erosion-sensitive geological formations, such as the shale units of the Shemshak Formation and the shale parts of the Karaj Formation in the Mosha-Fasham fault zone, under the influence of landslide processes, introduce a large sedimentary load into the river channel during heavy rainfall. In general, common forms of river canals in the region include canal bed sediments, inter Channel Islands, foothills and old alluvial terraces in the downstream areas (Lavasan). Examination of the longitudinal profile of the main part the river from the junction of Sarbandan and Shemshak tributaries downstream to the dam site showed that the middle part of this section of the river is affected by a sudden change of slope due to the Mosha Fasham fault. The changes in the bed level and the increase of the mentioned slope in the river route are due to the function of fault displacements in the region and the entry of large volumes of gravel flows from the river valley walls into the river channel flow path. In the riffle of Jajrood River, Lavasan Municipality has implemented dams to beautify and build a park for tourism that have upset the balance of the waterway. This raises the bed level upstream. Besides, river water quality has also been affected due to time delays in water flow.