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Sediment-laden flow and chemical-constituent concentration data from erosion plots and small watersheds are often needed for erosion and water-quality impact evaluations and modeling. The original drop-box weir was modified to pass the design flow through the V-notch and box sections of the weir. Approach conditions consisted of gutters and aprons that divert runoff water and sediment to the box at right angles. An evaluation of the effects of these approach conditions on the original rating for the drop-box weir and the effects of not having a false back wall for erosion-plot use was conducted. A rating for low flows was obtained. The rating for the drop-box weir with the new approach conditions was different from the original rating. Three rating equations were fit to the data for the three flow ranges. Observations of weir performance suggested that the reason for the three equations was related to the stepped side weirs. This study and other published studies led to the conclusion that the drop-box weir should not be modified unless a new study of the effects of the changes on the rating are evaluated. The gage height for the design flow for sizing the modified drop-box weir was established. A decision table was developed for appropriate use of the drop-box weir.