زمین‎لغزش بزرگ سیمره و توالی پادگانه‌های دریاچه‌ای

Introduction
The Landslide dams are the natural result of blocking of river channels by the mass movements. The effective factors of landslide dam formation include: the shape of valley in relation with the geometry, volume of debris, the characteristics of landslide debris, river discharge. The dam lakes are formed by landslide in the mountain valleys of Zagros and Alborz. Behind these dams, sediments were deposited simultaneous formation of the lake. After destruction of dams, several terraces are formed along the valleys. Investigation of the sequences, strata and streams are proxies to determine the tectonically and climatically changes of the past. The lake terraces can be formed by erosion of coastline, sediment accumulation, or a combination of these processes. These landforms are formed in various points of Iran. In the study area, blockage of Saymarreh River by Saymarreh landslide led to formation of a lake behind the landslide in the past. Primary field investigation revealed the several lakes and river terraces along the Saymarreh and Kahskan riverbanks and in the upstream Saymarreh landslide mass. The purpose of this study is the determination of the extent and sequence of these terraces and relation of their morphometric characteristics with the process of formation of large Saymarreh landslide.

Methodology
The used method in this research is the analysis of data that accrued based on fieldwork study. Therefore, we used the satellite images, topographic and geologic maps, SRTM10m and ArcGIS software. Accordingly, we measured height and distance of the terraces from active riverbed and their boundaries and area. Then, morphometry of these landforms was analyzed through drawing the cross sections, profiles and integrating sedimentary data.

Results and Discussion
Saymarreh landslide has a large landslide mass in which length is more than 15×20 Kms; its volume is more than 40Gm3, which had occurred along the Saymarreh River in the southeast of Ilam province. The evidence of this survey showed that a strong earthquake can be occurred, joining Saymarreh and Kashkan rivers in front of this landslide and their undercutting and excavating the base of landslide are among the main causes of its instability. According to the evidence after the landslide, the following subsequences occurred: first, because of the flow of the landslide mass, the course of Kashkan River was changed; second, in the process of obstruction of Saymarreh and Kashkan rivers and formation of landslide dams, the little width of the valley (7.25 kms), the great volume of the landslide mass, the intensity of the landslide and the size of the landslide sediments and the little shear strength of Saymarreh river are the most important factors of formation a vast lake behind this landslide. The great volume of extant lake sediments and their sequences showed a large and stable lake environment for a long time. The first depletion canal had been excavated next to Helosh and Chenareh buried anticline's. This phenomenon is derived from the function of Helosh and Chenareh anticline's that had performed like a dam and caused compaction, a rise in height, and the stability of landslide sediments against the lake water.

Conclusion
The research findings show that the depletion of the lake in the course of Saymarreh River had occurred during several stages. The comparison of the height of river and lake terraces shows a direct relation between them. The comparison of terrace height first (635m), second (595m), third (585m) and forth (575) with height of incision surface first (610-640m), second (590-610), third (570-590m) and forth (550-570m) respectively indicated that the formation of each terrace surface are related to cutting cycle of the landslide mass by Saymarreh river. By decreasing base level as a result of excavating landslide mass, the Saymarreh river had continued excavating lake bed until it had reached its old bed and formed a terrace surface. After this stage and cutting of lake sediments and approaching its old bottom, it started sedimenting alluvial at the lake terrace surface. This process had occurred many times and led in the formation of several terrace upstream this landslide mass. Of course, the thickness and volume of the terrace is not same. This difference is the result of the height and volume of the landslide dam and the stability of the lakes according to the chronological order. Therefore, reoverlaying new lake sediments on the old lake and river sediments (rhythmic sequence) are due to forming Lake Environment again. The findings of surveying sediments sequence of lake terraces indicate that the large Saymarreh landslide had been unstable during some stages (3-4 stages). Great extent and thickness of the older lake terraces indicate that the first landslide had a greater extent and the volume of sediments indicates its longer stability. The Less thickness of the new lake terrace and their overlaying on the old sediments indicate that the later landslides had less volume and extent, and a shorter stability.