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applications. For example, use of the Manning equation, assuming uniform flow, or even the stage-discharge approach, can result in predicted water surface elevations higher at downstream stations than at upstream stations. This may be caused by errors introduced during data collection, or by failure to account for changes in the bed elevation adequately. This problem can result in the appearance that water is running uphill, cause for alarm in engineering work. This error is real and reflects a mistake, but in respect to depth, this error is usually small. If sections are 100' apart or more, an error of tO.05 feet is acceptable; if outside this range, additional data collection may be warranted. The following section deals with the more-serious. limitations of the various approaches which can have an effect on the accuracy and reliability of the predicted hydraulic conditions and, therefore, on the instream flow recommendation itself. GENERAL LIMITATIONS IN HYDRAULIC SIMULATION The major problem encountered with the types of hydraulic simula- tions discussed in this paper, is that predictions of hydraulic conditions are made using as few discharge measurements as possible. Over a wide range of flows, certain processes occur in the natural channel which affect the relationship between stage and discharge, as well as the velocity distribution. These processes may introduce significant errors into any hydraulic prediction, regardless of the method used. Several characteristics of open channel flow and factors affecting the stage- discharge relationship are discussed below. Factors related to on-site channel conditions will be discussed in greater detail in the section on site selection. Extended over a very wide range of flows, from essentially zero flow to overbank, many channels will exhibit an S-shaped rating curve such as that shown in Figure 7. As the channel fills from a zero flow stage, much of the discharge is accomodated by increasing the width of channel filled with water. After the channel width is essentially filled, increases in discharge are accompanied by well-behaved increases in stage. If the flows of interest lie along this straight portion of the rating curve, accurate predictions are possible with relatively few measured discharges. As the discharge overtops the banks, the additional flow is again reflected in an increase in width (and velocity), and the increase in stage is proportionately less. The degree to which changes in the stage-discharge relationship occur is primarily a function of channel shape. If the range of flows of interest for instream flow studies includes extremely low or high flow conditions, the investigator should be prepared to collect additional stage-discharge data as required to define the rating curve under those conditions. A related phenomenon occurs in alluvial channels (a channel cutting through previously deposited materials; primarily, those streams having unconsolidated gravel, sand, or silt beds). In these streams the bed material is often moved by the stream, especially at the higher dis- charges. These streams often exhibit a stage-discharge relationship 23