عنوان مقاله [English]
denderogeomorphology is using trees as natural archives or "silent witnesses" . denderogeomorphology refers to dating of tree rings science that studying the annual growth rings of trees by measuring the number and change in thickness of the ring that create. But ‘dendrogeomorphology’ is one of the subﬁelds of dendroecology and has been widely used to study and date past geomorphic processes. dendrogeomorphology review the spatial and temporal aspects of the surface processes by analysis the annual growth rings and growth form of trees. Also enables the reconstruction of events with annual and sometimes even seasonal precision. In this paper, we attempt to study and analysis the erosion and deposition processes on the Neka river channel by using dendrogeomorphology.
With approximately 3 km length, the study area is part of the mainstream Neka River in the southern of Neka city. The beginning of study reach is located in 36˚ 37ʹ 24˝ northern latitude and 53˚ 21ʹ 52˝eastern longitude and its end terminated at Abloo convert dam and the exiting place river of mountain.
This research method is experimental- historical (Laboratory-survey and analytical). First the basic information was collected and done the primary survey of region. Then analyzed the Neka river water and sediment discharge data in Abloo hydrometric station and prepared the base maps and geomorphological map. In this study we sampled the exposed roots of trees in the bed and river margins for the reconstruction of river side erosion and from buried tree stems in depositions to the reconstruction of deposition and dating the alluvial terraces until to reconstructed their age by change in the width of growth rings. Sampling method is selective and during a survey the samples selected and their Coordinate was recorded by GPS and 7 samples of tree roots and 51 samples of tree stem were taken using the increment corer and hand saws to the state core and disk. Then we according to change in growth pattern from concentric to Eccentric growth determined the year of root exposed. We prepared an identification card for each tree sample in during the sampling and all information about the sample. Finally, after preparing the samples, the number of tree rings as well as ring width using digital table LINTAB attached to the stereomicroscope and TSAPWIN software program were counted and measured with an accuracy of 1: 100 mm. By using this program, a tree rings were sorted and samples growth curves were drawn and then analyzed and reconstructing dating erosion and deposition in river margin. Then results of dendrogeomorpholology studies compared with Water and sediment discharges data and the relationship between them was established.
The analysis of growth trend graphs in exposed root samples represented useful information about the first year of root exposed event. But since the total amount of erosion in fluvial geomorphology at bed and margin the river may occur during one or two flooding, therefore it may be wrong (opposed to the slope) that estimate annual erosion rate for banks of river bed through the exposed roots. Because it may be wash the total eroded soil on the terraces or on bed margin only during at one flood event and calculation the annual amount of erosion to be wrong.
The analysis of growth graphs demonstrated that Floods 2003, 1999, 2008 and 1996 year, respectively, have most important role in the tree roots exposed and among 7 studied roots, 2003 is the first year of exposed tree root in 3 of them and this exposed is related to flooding at this year with 361 m3/sec discharge. Two roots in 1999 is effect of flood event at this year with 2000 m3/sec in discharge, one root in 2008 in effect discharge of 130 m3/sec and one root in 1996 and in effect flood event with 335 m3/sec in discharge exposed in these years. Of course, being more exposed roots in 2003 could be result from young age of trees in margin the river bed and certainly if there were older trees on margin of bed, there was possible to reconstruct the older date from erosion and deposition.
But among the 51 taken sample for reconstruction age of alluvial Terraces, the age of 34 tree sample was according to the age of the alluvial terraces that tree grown on it. In other words, the tree is as old as alluvial terraces. Since the floods carry large deposits with self and deposited them in the appropriate places. Therefore was forming a new alluvial environment and or new terraces at during this process that Provides an appropriate place to tree grow and considering that tree grows in the case study is very fast so with determining trees age that one or two year after flood event grown over the alluvial beds, we can to determine approximately the age of terraces that tree grown over it with accuracy of ± 0 to 3+ years. According to the results 7 alluvial terraces were determined and reconstructed. The oldest terraces reconstruct has 33 years of age in the study area that in 1983 formation due to the peak discharge event in dated 20/7/1362 and after that is 29 year age terraces. Also other terraces were reconstruction and determined with the age of 20, 17 and 13 years in the area. The youngest terraces in this study have 6 and 7 years ages, according to the tree ring data. Of course, also there may be younger terraces but due to the short age of the trees grown on the terraces and large variations in their growth rings width, there is possible thatthe determination of these terraces according to the tree rings along with be wrong.