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Flow monitoring at watershed scale relies on the establishment of a rating curve that describes the relationship between stage and flow and is developed from actual flow measurements at various stages. Measurement errors increase with out-of-bank flow conditions because of safety concerns and difficulties in measuring flow velocity in the flood plain. With increasing frequency of large rain events associated with climate change, it becomes critical to have an accurate rating curve for out-of-bank flows. We propose a methodology based on the Manning formula on one hand and physical and mathematical principles that govern the hydrograph and the rating curve on the other hand to test and develop the high flow limb of a rating curve. The methodology was developed and tested using flow data from the Goodwater Experimental Watershed, a 72-km2 watershed in Northeast Missouri. A final water balance on flow events was performed using measured precipitation, and evapotranspiration and soil water estimated from a SWAT model. Preliminary results indicate that flow values that correspond to out-of-bank stage were overestimated by 30% in the original rating curve. This method provides additional and innovative means to verify the accuracy of the high limb of a rating curve.