With the development of economy and social services, increased need to reduce risks, control risks and other important measures in order to provide program management and follow-up plans vulnerability, Having the right information and understanding the current situation in the field is essential for prevention and planning measures, Therefore, research on risk reduction and knowledge of threats in the Arangeh region is essential, as one of the areas tourist attraction regions in Karaj's catchment area.

Geomorphology of River studies landforms and processes of river and predict changes using models and field studies and laboratory. And new analytical tools and techniques, growing and expanding with the help of river engineering.

This eventually leads to gain new capabilities in the field of river management, landscape restoration, risks and geomorphological studies ancient river.

In most cases geomorphological processes that are created by river systems, are causing environmental hazards of natural and human environments. In this paper, we have investigated the risks of geomorphic processes, especially risks of flooding and river flooding and is calculated for the maximum flood discharge for subarea also. In this article, it has been found that most of the flood will be calculated based on the map of the geomorphology of the area and the discharge sub basin. The purpose of this study, is assessing damages caused by the flood risks in the area. It is obvious that the results of this study will enable the pre-crisis phase of the crisis management system and can help to tourism and physical planning in the area.

Arangeh basin is an area of 10,090 hectares and a maximum height of 3665, at least 1637 m and average height of 2689 m. Arangeh area have an annual precipitation about 785 mm. Arangeh watershed is located within the northern city of Karaj, 15 km Karaj Branch, Karaj Dam east side of the river and inferiors (Amir Kabir).

In this study, to analyze the flood in the basin, a variety of sources are used including surveys of library data and documents, topographic base map scale of 1: 25,000 geological map of 1: 100000 taken from the ground geological, climatic data obtained from meteorological Organization, hydrological data obtained from regional water Alborz Landsat satellite image.Also field visits, the use of GPS and GIS software Arc GIS Version 10 was main parts of the survey.

The calculated concentration time by Krpych method to estimate the flood of data base, then estimate is based on a regional analysis of runoff and peak discharge of flood.

According to Hydrogeomorphic properties basin unit (sub-21) has the maximum flood discharge which is mostly covered by alluvium and located on the ground impermeable siltstone, waterways due to morphological features steep, mountainous dominant morphology, concentration time low basin, poverty and lack of vegetation (about 15 and 50 cubic meters per second in the 50 and 100-year return period). Other sub-basin with high flood discharge of sub No. 3, 5,7,9,12,14 and 16 are in Central, East, North, East and South of the basin villages.

Many parts of the Arangeh basin has slopes of more than 60%, which is an important factor in the effect of runoff, reducing the time of concentration, poor soil and vegetation and is an important factor aggravating flood risk and erosion. The presence of vegetation in these areas can have an important effect in obstructing runoff, reduce the rate of runoff, reducing flooding and consequently the reduction of soil erosion. We can largely control the flood basin watershed management practices and proper management range in the above units.