Human water resources are increasingly faced to destruction threat because of many reasons. Among the most important water resources, the rivers due to open systems are more vulnerable to these threats. One of important things in water usage is the water quality which limited its application for various purposes. This study has been used IRWQIsc water quality index to evaluate water quality of Tarwal river in Kurdestan province, which this index has been determined for measuring the quality of the water resources in Iran. Measurement parameters include: BOD, DO, COD, EC, pH, NO3 and PO4 which have been measured in twenty monitoring stations in the winter, because the river over hydration in this season. IDW interpolation method was used for zoning of index evaluation results. The results of index evaluation showed that a large part of river placed in bad and very bad quality class, which shows the status of the poor quality of the river. The reduced water quality may be caused of the discharging the industrial, agricultural and rural waste waters into the river and reduced river flow because of consecutive drought in recent years. IRWQIsc water quality index and IDW interpolation method were appropriate for determining the water quality of Tarwal river.
