Measurement of pollutant loadings is important in any lake management program. The flow interval method is among the most widely used today for this purpose. However, the Baltimore City Water Quality Management Office found this methodology to be extremely sensitive to the unit of flow (e.g. daily average, 15 minute discrete records) used in the computation. In three tributaries monitored, errors attributable to the flow unit used in phosphorus and sediment loading computations were found to be as high as 44 percent and 71 percent, respectively. Potential errors are greatest for streams with flashy hydrographs and a strongly positive flow/concentration relationship. The degree to which phosphorus and sediment loads are underestimated by using daily averaged flows varies with watershed, hydrologic year, and flow-concentration dynamics.