طبقه بندی ژئومورفولوژیکی الگوی آبراهۀ طالقان‎رود در محدودۀ شهرک طالقان از طریق روش رزگن

Introduction
The classification of natural streams is not a new approach in geomorphology. Over the past 100 years, there have been about 20 published methods about stream classification systems. The first recognized classification was by Davis in 1899. Davis classified streams in terms of age (youthful, mature, and old age). The classification systems devised between the years 1899 and 1970 were largely qualitative descriptions of stream features and landforms and difficult to apply for all rivers in the world.
Primary efforts in Rosgen classification began in 1973 and the preliminary version was introduced in 1985 to the scientific community. Rosgen classification includes four levels. Level I is a geomorphic characterization that categorizes streams as “A”, “B”, “C”, “D”, “DA”, “E”, “F”, or “G”. Level II is called the morphological description and requires field measurements. Level II assigns a number (1 through 6) to each stream type describing the dominant bed material. Level III is an evaluation of the stream condition and its stability. This requires an assessment and prediction of channel erosion, riparian condition, channel modification, and other characteristics. Level IV is verification of predictions made in Level III and consists of sediment transport, stream flow, and stability measurements. Taleghan River is in the vicinity of Tehran, Capital of Iran. For some economic considerations and that the position of this river upstream of dam reservoir, to increase life of the dam, it seems important to classify this river for planning issues.
 Watershed of Taleghan River is one of the major basins in Sefidrood River Basin in the southern slopes of Alborz Mountains, located in North West of Tehran. This basin is located from 36◦ 5' 31" N to 36◦ 23' 37" Nto N and from 50◦ 21' 00" to 51◦ 1' 16.
 
Materials and Methods
In this study, Rosgen classification at level I and II have been conducted. For this, some data including topographic map 1:400 scale of Taleghan has been prepared by the Tehran Regional Water Organization, image of Google earth to determine the overall plan for the river channel patterns and the field studies to determine the bank and increase the accuracy of data grain size. Initially to determine the level I, using satellite imagery the area was divided into three parts based on similarity of shape, then channel patterns (single thread, multiple thread channels) and channel shape (narrow- deep/ wide- shallow) was determined based on these images and fieldwork. Channel slope in each section was also determined. For level II classification, some parameters are required to be studied, that were obtained from cross section for each reach. These are including entrenchment ratio, width/depth, sinuosity and channel slope. Finally, size of the material was used. These data were obtained from Regional Water Organization.
 
Results and Discussion
The results indicate that the classification of section A is arterial, wide and shallow channel, the slope is less than 4%. The other two have direct channel model with the narrow and deep beds, the slope is less than 4%. In the A section is of type D and the other two parts have the pattern B. The results of the classification of level II the calculation parameters in section A are the D3 and in two other sections the B3c.
 
 Conclusion
David Rosgen for each river type has expressed specific managerial interpretations. The obtained results suggests: river upstream (4.5 km above the river) have high degree of sensitivity to disturbance, poor recovery potential, too much sediment supply, high erosion potential, and moderate ability in control of vegetation. The two lower sections (2.8 km bottom the river) have low degree of sensitivity to disturbance, excellent recovery potential, low sediment supply, low erosion potential side, and also moderate ability in control of vegetation. Application of these interpretations can be applicable in assessment of potential impacts and risk analysis and management issues.