For system analysis purposes one would in general require natural flow sequences with a relative high confidence rating of an A or at least and B. Confidence ratings of C and lower should preferably not be used. The natural flow data for the total Vaal River catchment as well as the Orange River catchment from Vanderkloof Dam and upstream falls in the A and B confidence rating with the exception of two small areas which will have a negligible affect on results. In the remainder of the Orange River catchment only two sub-catchments (Hardap and Naute) within the Fish River subcatchment has a B rating which is also regarded as a low B rating. All the rest of the sub-catchment has ratings of a C and less. This means that on a catchment area basis approximately 32 percent of the area has natural flow records with a confidence rating that is high enough to be used in system analysis. From this 32 percent of the catchment area, approximately 94 percent of the natural flow reaching the Orange River, is however generated. This brings along a total different perspective, showing that on a volume basis only 6% of the natural flow records for the Orange River are not at an acceptable confidence level. This 6% represents the runoff volume that is expected to reach the Orange River main stream and expressed as a volume it would be in the order of 680 million m³/a. This volume was not neglected in previous analyses, but is just not at the desired confidence level. This means that the margin of error is most probably higher than the 10% to 15% generally accepted for hydrology and might be in the range of 25% to 30% (170 to 200 million m³/a error) for 68% of the catchment. Improving these flow records will contribute to an overall improvement in accuracy of approximately 1% which is not much. Considering only the Lower Orange and Fish River inflows the improvement might be in the order of 12%, which need to be taken into account when the focus is on developments in the Lower Orange and Fish River sub-catchment.