The purpose of this paper is to build a simulation model by using HEC-RAS software to simulate the reality of water movement in the main river of Basra City (South of Iraq) which is known as Siraji-Khoura River. The main objective of the simulation is to detect areas where the water cycle is interrupted in some stations of the river stream, as this river has become an outlet for the disposal of sewage, leading to pollution and causing weakness in some sections of the river & obstructing the water cycle that takes place between this river and Shatt al – Arab river. A field survey data of the river and its banks were adopted to derive the grades, longitudinal and cross sections of the river, these data included three-dimensional coordinates observed by precise GPS device. Depending on the river's derived sections and elevation of water, a one dimensional unsteady flow model was constructed by HEC-RAS software to simulate the behavior of water flow during the tide periods. The results of simulation illustrated the weak areas that cause obstruction in the water flow within the river stream. It also determines the solid waste accumulation areas and stations where bed level rises from the water levels, causing the water cycle to break down in some parts of the river. Thus, an integrated database for the rivers of the study area was obtained, in order to the correct design decisions that are in the interest of these rivers.