Growing islands became noticeable phenomena in the channel of River Tigris within Baghdad City during recent years.Despite the fact that large amount of sediments are trapped in reservoirs on the River Tigris and its tributaries thenumber of islands are increasing with time. This is due to the debris of destroyed bridges in the wars of 1991 and 2003.As a consequence the ability of the river had been reduced to pass ood waves. This fact caused ooding parts of majorcities like Baghdad. Cross sections of the River Tigris were surveyed in dierent occasions (1976, 1991 and 2008). In1977 the survey was conducted by Geohydraulique and in 1991 by University of Technology - Baghdad (extended forthe previous study). The last survey was conducted in 2008 by Ministry of Water Resources extending 48 kilometersfrom Al-Muthana Bridge till the conuence with Diyala River at intervals having horizontal spacing of 250m. The datawas used to predict the maximum ood capacity for the river using HEC-RAS through performed a one- dimensionalhydraulic model for the ow. The average discharge of the river in Baghdad had been calculated for the past ten years.This value was introduced in the model. Then dierent scenarios were applied by increasing the discharge in orderto nd out the critical discharge that can cause inundation. The procedure continued to detect the areas that had beeninundated and the water level was recorded. The primary runs for the model showed a signicant reduction in thecurrent river capacity in comparison with what the river had used to hold during oods of 1971 and 1988. The threesurveys that had been conducted on the same reach of the River Tigris indicated that the capacity of the river to passwater had been decreased. In addition the changes in the morphology of the river cross sections were very clear.