خلاصة:
نیمرخ طولی رود یکی از مؤلفههای اساسی سیستم رودخانهای است. در این تحقیق نیمرخ طولی 15 رودخانه در البرز شمالی، که بخشی از حوضة آبریز دریای خزر هستند، بررسی شدهاند. نخست، با استفاده از مدل ارتفاعی رقومی، با قدرت تفکیک 12.5 متر شبکههای رودخانهای شناسایی و نیمرخ طولی آنها ترسیم شد. شکل نیمرخ طولی رود با شاخص تقعر (CI) و شاخص SLK اندازهگیری شد. سپس، شاخص گرادیان طولی رود (SL) برای هر قطعه از نیمرخ طولی محاسبه شد و با استفاده از نقشههای سنگشناسی و گسلها تفسیر شد. نتایج نشان میدهد که شکل نیمرخ طولی رودخانههای البرز شمالی در درههای عرضی بهصورت مقعر با چندین شکستگی است که بیانگر غلبة فرسایش در بلندمدت است. در درههای طولی البرز شمالی، نیمرخ طولی رودخانهها بهصورت محدب است که ناشی از ساختار چینخوردگی و بالاآمدگی سنگهاست. بررسی شاخص SL نشان میدهد که عامل سنگشناسی و گسلها بهترتیب 53 و 45درصد در ایجاد شکستگی در نیمرخ طولی این رودها نقش داشتهاند. از مجموع 98 خط گسلی، که بهصورت عرضی رودخانهها را قطع کردهاند، فقط بیستدرصد آنها موجب شکستگی در نیمرخ طولی رود شدهاند. اثر گسل در نیمرخ طولی رودخانههای غرب و شرق استان مازندران بهترتیب 70 و 30درصد بوده است.
Introduction The longitudinal profile of river is one of the main components of the fluvial system, which is result from the interaction between lithology, tectonics, fluvial incision and base level change. The longitudinal profile of the rivers that are in a Equilibrium state have concave form, but several factors cause changes to the longitudinal profile of the river, including, lowering base level, rock resistance, structural control, sediment input, non-fluvial processes, bed load effects, human changes and insufficient time. Alborz Mountains are considered as one of the geomorphological zone of Iran, which geological, climatological and vegetation characteristics have created a special geographic landscape. In this study, longitudinal profiles of the northern Alborz rivers (Mazandaran province), which are part of the Caspian Sea basin, have been investigated. The aim of this research is to quantitatively analyze the longitudinal profile of the rivers in the region. Methods and Material The longitudinal profile of 15 rivers in the northern Alborz were investigated. The drainage network was extracted by the 12.5-m DEM and Longitudinal profiles of the fifteen trunk channels were extracted using the ArcGIS 10.3. Geological maps were used to interpret lithology and fault distribution. The form of longitudinal profile was measured by the concavity index (CI) and SLK index. The longitudinal gradient index (SL) was calculated for each segment of longitudinal profile. concavity index was computed based on deviations from a straight line profile (Philips and Lutz, 2008). The SL Index analysis was performed with Hack's (1973) method.The SLK index for the longitudinal profile of the river was calculated by normalizing the distances in the horizontal axis and the elevation in the vertical axis. To normalize values (SLK), in the horizontal axis of the graph, the normalized distances are represented as (d / D), where d is the distance between the specific points along the longitudinal profile and D is the total length of the profile. In the vertical axis of normalized height (e / E) it is shown that e is the height of specific points along the longitudinal profile and E is the height difference between the beginning and the end of the longitudinal profile (Vojtco et al., 2012). The maximum amount of concavity along the SLK profile was determined as Zmax. Results and Discussion The northern Alborz rivers are not smoothly concave profiles and there are numerous knickpoints in their longitudinal profiles. In all cases, the best fit (R2) of regression has been matched with polynomial equations of degrees 2, 3 and 4. the coefficient of determination of these regression equations was very high (>0.96). Some of rivers such as Kheyrood, Kojur and Noor have a convex profile with concavity index of -4, -4, and -11respectively. A number of other rivers such as Tajen, Babol, Sardabrood and Chalakrood with CI 8, 7.5, 9 and 8 respectively have more concavity than other rivers. Based on the relative concavity index (CIrel), 20% of the longitudinal profile of the North Alborz in the convex form (CIrel Conclusion In the longitudinal valleys, the rivers have a convex profile, where uplift (active tectonics) is dominant. In these valleys, the rate of uplift is greater than the amount of river incision and the river can not create a equilibrium profile.The longitudinal profile of the rivers in the transverse valleys is Concave–convex with erosion steps that indicate long-term predominance of erosional processes. Their profile is due to the high altitude difference, short distance to the base level (Caspian Sea) and lithological resistance. It seems that river icision has overcome the tectonics and the lithological factor has more effect on the longitudinal profile of the Northen Alborz rivers.