the flood of december, 1964

introduction extreme floods, hyperconcentrated and debris flows are common natural hazards in the basins which located in the northern slops of alborz mountain range (iranian north system). flood occurrence in golestan national park in 2001, 2002 and 2005( hosseinzadeh and jahadi toroghi 2012), yearly flash floods in masuleh catchment( rezaei, 2009), large flood in neka basin in 1999(poorahmad 1999) and recent floods in seh- hezar basin are only a few examples. flood hazard analysis over iranian mountains is always considered scientific challenge because long discharge records are not available. on the other hand, flow gage stations may not correctly recored during large and extreme events because they are often washed out by large events or discharge exceeded the recordable level. where the systematic data is not available we need to understand the spatial and temporal occurrence of the processes with an approach geomorphology. dendrogeomorphology can provide an alternative for flood event date by analyzing the growth-ring series of trees affected by past extreme floods. dendrogeomorphology represents one of the most precise and accurate methods for the dating of various geomorphic processes ( alestalo, 1971; shroder, 1980, stoffel and bollschweilir, 2008, 2010 and 2012). however the term - dendrogeomorphology - was coined by alestalo (1971), how ever the theoretical background for the study of flooding and sedimentation processes was provided by sigafoos (1964) and harrison and reid(1967). helley and lamarche (1968) applied this techniques so as to determine the recurrence interval of flood events in northern california. later studies in the united states focused on floods in the white river (arkansas; bedinger, 1971) or high summer flows and floods in the potomac river (yanosky, 1982, 1983, 1984).dendrogeomorphology was also used for dating of old large magnitude floods by costa (1978). more over, george and nielson (2000, 2003) have analyzed anatomical changes (“flood rings”) in quercus macrocarpa to identify 19th century high-magnitude floods in the red river of canada. in europe, only a few studies have focused on floods so far. for instance the one published by astrade and begin (1997) documenting recent spring floods in the saone river of france (stoffel 2008). recently, scientists in europe are focused on determination of flash flood discharge in ungauged mountain catchments (ballesteros et al., 2011 and 2012, ruiz et al., 2010). some review papers in dendrogeomorphology have been published by shroder(1980), hupp(1988), stoffel and bollschweiler(2008,2009), bollschweiler and stoffel(2010), stoffel and willford(2012) after alestalo book(1970). study of floods using dendrogeomorphology has not been interested in iran so far, therefore this paper introduces a new method for flood hazard researches. therefore, the aim of this paper is to date of past large flood events in order to improve frequency analysis and hazard estimation. we focus on the identification of large floods based on growth disturbances in heavily affected trees located in or next to the channel of the seh-hezar river in the northern of iran and present (i) a geomorphic map of the floodplain and its channels (1:7000)) (ii) an analysis of the relation between geomorphology and external tree disturbances; and (iii) a tree-ring-based dating of past large flood events. study area the study area includes a multichannel floodplain with 1km length in the main channel of seh- hezar river located in the northern slops of alborz mountain. she-hezar catchment drains an area of 462 km2, originating from takhte-solayman glaciers and entering in to caspian sea (fig. 1). the altitude in this catchment ranges from 389 to 4650 m which causes forms of many different geomorphic- ecologic zones including rainy forest areas in the low altitudes to preglacial and glacial environments at high altitudes. the annual average temperature varies from 0 to 13 °c; and the basin is characterized by abundant precipitation which falls as snow at high elevations in winter. extreme floods are common in the seh-hezar river catchment and are triggered by heavy storms and high stream gradient, which makes extremely high discharge and debris transport rates possible. local organizations emphases that the occurrence of floods have been increased in the river by glacial melting which is accelerated under global warming phenomena during the last decade. some studies is been referred to acceleration of melting and ablation of alam-kuh and takhte-solayman glaciers by global warming(e.g, yamani 2009) hydrological data are available only for 10 years(2001-2011) from the “servash-poshteh” station located in the 5 km upper part of the study area. material and methods the materials were used in this paper consists aster digital elevation model, etm satellite image, maximum flood discharge records of servash-poshteh station and 20 tree samples as cross section and wedges. the methods applied in this study can be divided into three main sections, namely: (i) field procedures; (ii) tree-ring data analyses; and (iii) flood reconstructions. floods and hyperconcentrated flows may affect the morphology of a tree in different ways, leading to different growth responses in the tree-ring series (stoffel and bollschweiler, 2009). the most frequent disturbances and associated reactions in trees growing inside or adjacent to fl flood torrents in the study area is shown in fig. 4. floods may (i) tilt the stems of trees growing in the river, on gravel bars, or river banks through the unilateral pressure of the flow or through the impact of individual boulders (braam et al., 1987; fantucci and sorriso-valvo, 1999; stoffel et al., 2005); (ii) expose roots as a result of river bank or gravel bar erosion (lamarche, 1968; carrara and carroll, 1979; mcauliffe et al., 2006); (iii) remove the bark from the stem and injure the cambium from the impact or the abrasion of boulders and wood transported in the flow (sachs, 1991; larson, 1994; bollschweiler et al., 2008); or (iv) shear off trees and cause the formation of candelabra growth following severe impact by boulders (butler, 1987; shroder and butler, 1987). the dendrogeomorphological study followed a geomorphological survey of the study based on the recognition and mapping of channel landform and deposits (fig. 3) the sampling strategy was based on external evidences of flood activity in trees. we selected a total of 15 acer and alnuse that were influenced by extreme floods and were located within the channel, on the gravel bars, or on the banks of the channels which marked by a, b, and c in the study site. samples were dried in the outdoor area in a summer dry whether (mashhad city) and sanded with hard and soft sandpapers until got the clear surface to simple accounting of tree rings. tree rings were counted by a dissecting microscope with an accuracy of 1/10 mm. furthermore, samples were analyzed visually, and tree rings showing compression wood or callus tissue were noted in a dedicated file and flood events were dated using these wood changes. results and discussion in the present study, 15 acer and alnus trees affected by extreme floods have been analyzed with 20 wedge and cross sections to reconstruct extreme flood events at seh- hezar river (alborz mountan system) covering the past 100 years. while the time covered by this reconstruction is much shorter compared to other paleoflood hydrology methods (e.g. baker 2008, and benito 2005), our study clearly shows the potential of extreme flood analyses based on information contained in growth-ring series of affected trees. maximum annual discharges have been recorded from 2001 to 2011 systemically. the largest flood in this period with 6980 m3/s has been occurred in summer 2011.based on flood mark elevations on tree stems; this event approximately is similar to as large paleofloods in discharge and flood levels. a series of floods were recorded in 2003, 2004, 2007, 2010 with discharge or 5225, 2100, 2400, 2400 and 1850 m3/s respectively. the results from dendrogeomorphology analysis summarized in the table 1. these data shows occurrence of 6 extreme floods in the 1936, 1942, 1976, 1983, 1992 and 1998. the 1983 flood was buried the sampled stem trees in 1.2 m depth but 9 years later(1992) another extreme flood was degraded the sediments and exposed the new tree roots. consequently, the aggradation and degradation ages obtained by growth rings counting of the oldest spurting roots from stem and wood reactions after root exposure (fig. 10-3). the above mentioned root sample is scratched by flood of 2003 and the age of the flood is recognized by counting the rings in the callus tissue. the scars of 1976 flood are seen at the same level of 2011 flood and based on growth rings in the callus. we determined the age of this flood with seasonally precision (summer 1976). based on flood scar elevations on tree stems and channel characteristics, the paleoflood discharges is assessed. conclusion this paper clearly illustrates the ability of dendrogeomorphology for flood analysis and documents how dating of past events can be improved in areas where the lack of historical documents, rainfall, and discharge data prevents the use of traditional methods. this method can be used in the many mountainous watersheds in iran. comparing with other historical methods in geomorphology, dendrogeomorphology covers a short period of time but gives precise annual, seasonall and even monthly time for past occurrence floods in the ungauged basins. in this paper, we obtained the age of 6 extreme floods in the seh-hezar river and found out the amount of flood discharges was ranged between 5000 to 10000 m3/s (table. 2). in addition the frequency of extreme floods has been increased during the last decades with a concentrating in the summer season. it may causes by global warming which affected mountainous glaciers in the upstream of seh-hezar river. the results show more probable glacier melting in recent years (fig. 13).