AbstractDez River in Iran is a long river and has generally good-quality water. Unfortunately, in this river water quality has decreased over recent years due to drought, industrial and agricultural developments. Dez River plays a significant role in moderating the quality of Karun River After adjoining it. So, maintaining good water quality of Dez River is very important. In this case study, water quality index was evaluated in an 8-year period with regard to the growth of industry and drought occurrence. The calculated Iran Water Quality Index showed values of 70.1–85. This index reduced from extremely good to good for DEZ River. Simulation of some water quality parameters (electrical conductivity, dissolved oxygen, carbonaceous biochemical oxygen demand, NH4-N, NO3-N, and pH) were evaluated using Water Quality Model (Qual2k). The observed data in July 2012 and March 2013 were used for calibration and validation of the Qual2k model respectively. In conclusion, the Qual2k model was proposed as a suitable tool for water quality assessment and forecast in the future.