تحليل زيستگاه‌هاي حاشيه‌اي رودخانه طالقان بر اساس واحدهاي ژئومورفيك رودخانه‌اي

Riparian Habitats Analysis of Taleqan River Based on Geomorphic Units Survey and Classification System

واحدهاي ژئومورفيك، نمود فيزيكي فرايندهاي جريان آب و انتقال رسوب مي­باشند كه تحت تأثير عوامل تاثيرگذار در مقياس­هاي بزرگتر هستند. هدف پژوهش حاضر شناسايي و تقسيم­بندي واحدهاي ژئومورفيك رودخانه طالقان در بازه­هاي مختلف، براي تشخيص تراكم و غناي واحدها و نقش عوامل انساني بر تغيير واحدهاي ژئومورفيك است. با استفاده از مطالعه­­ پروفيل طولي براساس اختلاف شيب و ارتفاع، رودخانه طالقان به 3 بخش بالادست، ميان دست و پايين دست با ميانگين ارتفاعي 1840، 1920 و 2000 متر و اختلاف شيب 17، 11 و 3 درصد تقسيم شده، سپس در هر بخش، بازه­هايي به­منظور مطالعه واحدهاي ژئومورفيك با استفاده از روش GUS مورد بررسي قرار گرفت. شاخص­هاي مورد بررسي شامل تراكم و غناي كل واحدهاي ژئومورفيك رودخانه در هر بازه ست. زيرشاخص­هاي مورد بررسي شامل تراكم و غناي واحدهاي مطالعاتي دشت سيلابي و كانال پايه، تراكم واحدهاي ژئومورفيك در 5 واحد مطالعاتي جريان اصلي رودخانه، رسوبات رودخانه­اي، پوشش گياهي ميان كانال، دشت سيلابي و منطقه آبگير دشت سيلابي است. بر اساس نتايج، غناي واحدهاي ژئومورفيك در واحد دشت سيلابي و كانال پايه در بازه­هاي بالادست ((R=0.28 بيشتر و تراكم واحدهاي ژئومورفيك در بازه­­هاي پايين دست رودخانه (D=133.57) بيش از ساير بازه­هاست. در بازه­هاي بالادست به دليل تنش هيدروليك، تراكم واحدهاي ژئومورفيك كمتر بوده و امكان تشكيل زيستگاه­هاي فيزيكي در ميان بستر وجود نداشته اما به دليل تاثير كمتر عوامل آنتروپوژنيك پايداري واحدهاي ژئومورفيك بيش از ساير بازه­ها است. در بازه­هاي پايين دست به دليل كاهش شيب تنش هيدروليكي و افزايش انعطاف پذيري اكولوژيك، شرايط مناسب براي ايجاد لندفرم­هاي تجمعي بيوژئومورفيك در ميان بستر رودخانه وجود داشته اما اين واحدها در مقياس كوچك هستند زيرا تاثير عوامل آنتروپوژنيك، تغيير كاربري اراضي و برداشت شن و ماسه بخصوص در پايين دست رودخانه سبب كاهش زيرشاخص غناي واحد دشت سيلابي شده و اين امر تاثير منفي بر ايجاد روزنه فرصت و لنگرگاه گياهي به­منظور استقرار پوشش گياهي وزيستگاه­هاي فيزيكي داشته است.

Introduction The study of the spatial scale of river geomorphic units, along with their hydraulic elements, is suitable for assessing the relationship between ecology and river physics. In the present study, the classification of geomorphic units of Taleghan river and the comparison of different reaches of the river has been considered in order to analysis the condition of riparian physical habitats of the river. The results of this research can be applied for processes management of river considering the riparian ecologic situation of the river. Case study Taleghan river is located between 36°23′and 36°06′ N latitude and 51°10′ to 50°20′ E longitude. In this research, 7 reaches were investigated based on the difference of slope and elevation (Fig. 1). Methodology Recently, new approaches for river management have been developed in the REFORM project (REstoring rivers for effective catchment Management) funded by the European Commission within the FP7. In this project, a set of hydrogeomorphological assessment methods is defined by the stages used to assess the river conditions (Rinaldi et al, 2015 and 2016). The geomorphic unit classification and evaluation system (GUS) integrates these methods and utilizes it to classification, analysis and monitoring the set of geomorphic units at reach scale in the 3 space scale of the macro unit, unit, and subunit (Belletti et al, 2017). In the GUS method, the indices of geomorphic unit density (GUSI-R) and geomorphic unit richness are calculated. The designed sub-indices in GUS metho are the richness sub-indices of bankfull channel geomorphic units (GUSI-RBC), floodplain geomorphic units (GUSI-RFP), density sub-indices of baseflow channel geomorphic units (GUSI-DC), emergent sediment geomorphic units (GUSI-DE), in-channel vegetation geomorphic units (GUSI-DV), riparian zone geomorphic units (GUSI-DF) and floodplain aquatic zones geomorphic units (GUSI-DW). The definition of geomorphic units is based on the 3 levels of broad, basic and detail. The characteristic of macro units and units (in broad and basic level) were calculated using ArcGIS software and remote sensing data by Ultracam 10 cm satellite images. The detailed level was also studied by a combination of field studies and remote sensing. Results & discussion The total number of geomorphic units and their density in the A and B reach are lower than the probable units in other reaches. At A and B reaches, the lowest area is observed in the "emergent sediment units". In the upstream part of the river, the process of riverbank erosion is dominated, therefore sediment deposition around the bed or across the river channel is much less. The density of biogeomorphic units in the middle channel (C and D) reaches and any opportunity for the creation of cumulative biogeomorphic landforms at the macro scale is limited. In the case of biogeomorphic cumulative landforms establishment, these units are not stable due to the hydraulic stress gradient. Most of the richness of bankfull and floodplain geomorphic units are observed in the D and E reaches. The density of in-channel vegetation unit in F reach is lower than C, D and F reach. The highest rate of the richness of geomorphic units is observed in D, E and F reaches. The width of the river bed in these reaches is greater than A and B reaches. The highest rate of the density of in-channel vegetation unit was observed in the H reach. The number of vegetation islands in this reach is 30, which is greater than all the reaches. Suitable conditions for the creation of biogeomorphic cumulative landforms in this reach is more than other reaches. The aquatic vegetation unit is only visible at C reach. Conclusion There is a variety of geomorphic units and physical habitats in the downstream of the river as well as in the midstream compared to the upstream of the river. Although environmental aspects have not been studied in this research, these reaches have a habitat diversity due to decrease of the hydraulic stress gradient and increase of ecological flexibility. Although the mentioned diversity may not be important for macro-fauna and flora because the habitats have a small scale in most cases but they are significant for diatoms, algae, and smaller fauna and flora. The impact of anthropogenic interference, landuse change, and sand removal from the river bed, especially in downstream of the Taleqan River, reduced the density of the floodplain unit, and this has a negative impact on the formation of sedimentary islands and riparian vegetation (F reach). Anthropogenic factors have a negative impact on riverbed deposition and disturbed natural evolution of riparian and in-channel vegetation. Excessive deposition by sand removal destroys opportunity windows for plant anchorages and, in some cases, buries in channel physical habitats.