In this study we present a methodology of neutrosophic logic in defining river water quality of Yamuna, one of the most important river of Delhi. The laboratory test results of water quality parameters are not always found to be same in all locations of the river rather varies with wide range of interval daily, monthly and seasonally. For an engineer it is very difficult to define the water quality with degree of certainty. Thus it is quite normal for an expert to hesitate in his decision and feel more comfortable to express their views in the form of linguistic & hedges viz. 'good', 'very good', 'low pH', 'less turbidity', 'high BOD', etc. to list a few only out of infinity. Such types of imprecise data are fuzzy in nature and uncertainty is an integral part of evaluation of each data. As a result the experts perceptions are not found 100% truth rather a combination of truth, indeterminacy and falsity. To minimize the uncertainty from the perception result of expert we present a methodology of neutrosophic logic in present case study and define the river water quality of Yamuna more precisely.