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The stage-discharge relationship for Oak Creek near Corvallis, Oregon, is given in Figure 3. To determine the stage for any cross section, a least-squares equation is determined from a log-log plot of discharge against stage. For any interpolated or extrapolated discharge, the stage is calculated directly from this empirical equation. PREDICTING THE VELOCITY DISTRIBUTION If the velocity distribution is measured for each flow of interest, the data can be used directly and no analytical procedure is-needed to estimate the velocity distribution. In most cases, only a limited amount of resources is available to do field work in any particular i nstream flow study; hence, estimates must be made of the velocity distribution of flows for which velocities were not measured. Velocity predictions are made using techniques which are similar to those used to predict stage. However, for any discharge there is only one stage, whereas the velocity varies from place to place across the section. It is important here to define what is meant by a velocity distribution in instream flow studies. Figure 4 illustrates two ways of expressing the velocity distribution in a channel. Figure 4a shows the distribution as a. series of contour lines connecting points of equal velocity. Figure 4b shows the velocity distribution as a series of mean velocities in a group of adjacent channel subdivisions. The conceptualization of the velocity distribution for most instream flow studies is the type shown in Figure 4b. Essentially, each subsection or channel segment is treated as a separate channel, with its own depth, substrate, and average velocity. Any number of subdivisions may be used to define the velocity distribution in this manner; the more channel segments, the more detailed the description of the velocity distribution. In actual practice, around 20 subdivisions are most commonly used, although.there is no firm limitation to this number. In the following discussions, approaches to estimating the velocity distribution in a. cross section are described. The first section describes the use of the Manning equation where no velocity measurements are made to calibrate the equation. The second section discusses the calibration of Manning's n with a series of measured velocities at one flow. The third section describes a procedure using more than one set of measured velocities. MANNING EQUATION WITH NO VELOCITY MEASUREMENTS This approach requires the stage-discharge relationship to be known from the previous computation procedures. Other data requirements include the dimensions of the cross section and the slope (Sh if uniform flow assumption is made, Se if gradually varied flow). 15