The water quality of the Duhok Valley stream was studied as one of the sources of pollution for the water of Mosul Dam Lake, which is located on the Tigris river.The necessary environmental tests were conducted for the water samples taken from different locations in the valley to investigate the degree of pollution of its water using the water quality index (WQI), and the extent of the impact of self-purification processes in improving the water quality before it reaches the dam lake.The results indicated that the water quality of Duhok Dam site (S1) was of medium water quality, while the rest sites of the valley were of poor quality, as the water quality index ranged between (26.8 to 39.4) and this deterioration in quality is due to the huge amount of the wastewater drained into it, which led to an increase in the organic load, phosphate and EC25 to reach (66.0, 25.0) ppm and (1135) µs.cm-1, with a lack of dissolved oxygen in the water in some locations and for some periods, and no noticeable improvement was observed in the water of the valley before it reached the Mosul Dam lake except for dissolved oxygen, sulfate and chloride ions.