The citizens of Basrah city are suffering from the problem of high salinity water supply. This problem is proposed to be solved by installing RO units in 16 water treatment plants of Basrah city. RO units produce high-salinity brine water. The aim of this study is to predict the environmental impact of RO units installation measured in terms of TDS increase of Shatt Al-Arab River water. The TDS increase was predicted using HEC-RAS software. The software was applied to simulate twelve cases of TDS distribution in water of Shatt Al-Arab River. During each of the twelve simulation cases, the proposed RO units were assumed to have recovery percent (RP) of 50% and 85%. The simulation results indicated that when the proposed RO units have RP of 85%, the max percentage of TDS increase is 5.9%. While, when the proposed RO units have RP of 50%, the max percentage of TDS is 2.7%. The comparison of these results with the regulation criteria of brine water discharge shows that; when the installed RO units have RP of 85%, the percentages of TDS increase will exceed the salinity limit of most regulation criteria (5%). (C) 2016 Elsevier B.V. All rights reserved.