Rate of lateral channel migration along Lavij River Based on Chronology of the Trees along the river of Chamestan in Mazandaran

Channel location changes of the river is an important process in the river ecosystems and is a threat for human activities in flood plains. Exploring the how and why of these changes has an important role in predicting the process of future changes and channel migration order to manage the flood plain. On the other hand, human interventions such as sand removal from river bed, land use changes, river engineering projects, etc. cause changes in the pattern and direction of the river channel. Exploring the how and why of these changes has an important role in predicting the process of future changes and channel migration order to manage the flood plain. Each year, several significant and insignificant floods happen in Lavij River, which has led it face several lateral channel migration. The effects of this migration can be significant in the form of root exposure and Falling trees on canal margins, road demolition, agricultural land, stand bridges, etc.Lavage River basin area is 93 square kilometers of North Alborz independent basin located on the northern slopes of Alborz Range south of the city of Chamestan. This basin has a north-south direction and after leaving the mountainous area and crossing the agricultural lands and forest park Noor to the Caspian Sea. The Lavage Basin is bounded on the west and south by the Glendrud Basin and on the east by the Waz Basin. In this research, the evolution and lateral channel migration of Lavij River has been studied with the Dandrogeomorphology Technique.This study is a combination of field and laboratory methods that include field surveys and sampling, sample preparation and macroscopic measurements. After collecting baseline data from the study area, 4 intervals for field sampling Selected at each interval, were first mapped using river cross-sectional mapping and positioning of river channel geomorphic forms including active canal, abandoned canal, flood plain, point bars and terraces. In this method, the age of the trees in the flood plain, abandoned channels, terraces, point bars, and the location of surfaces in relation to each other are calculated; the lateral channel changes have been Reconstruction from the past until this day. Comparing the hydrology and chronology data of the present trees in different layers show that the channel migration occurred in the reach No 1 1374 and 1375, in the reach No 2 1 in almost at 1345 and 1346, and in the reach No 3 in almost at 1380. In the reach No 4, a significant incision occurred in the river bed and the major channel migration in the flood plain surface was in the left direction of the current. Basic channel migration is mainly related to flows that are less likely to occur, with longer return periods. study of the trees chronology data in the study area showed that the year of migration or abandonment at all intervals was not consistent with large floods with a long return period, but even in floods with a return period of less than 4 years. In the study area out of the 4 intervals studied in three intervals, the diversion of the river channel was caused by flood currents of less than 20 m3 / s,therefore flows with a return period of less than 4 years. But when such currents occur alongside transverse currents, the probability of shear and bank erosion is very high. That is, in subsequent years, larger discharges will manipulate the channel created in these floods until another severe flood occurs and lead to major migration in the stream and channel. In fact, exceptional discharges along with rare floods at the time of occurrence temporarily cause shorter and wider canals because geomorphic processes can respond to severe floods of several decades, river measurements (channel morphometry) will be reflecting stream history. The study of channel migration, with the chronology of alder trees in the study areas, showed that the major floods of the years 1345, 1346, 1374, 1375, 1376 and 1380 have extremely affected channel changes and the age of establishment of alder trees illustrates this.The floods of 1996 and 1997 have been recognized as an important event in the interval changes over the past few decades. These floods have led to the straight and narrowing of the channel (interval 1). These processes deepen the channel and bring the floodplain to a higher level. The geomorphic changes that occur will increase the amount of flow required to exit the channel and extend to the surface of the floodplain .