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DISCHARGE MEASUREMENTS The discharge at the time of the field measurements may be obtained directly if a USGS gage house is nearby, and if one can be relatively certain that the flow in the study area is the same as at the gage (i.e., no significant inflow or outflow between the gage and the study area). If access to a stream gage is not possible or the stream flow is not known to be the same between the gage and the study area, the discharge must be measured at the study area. Velocity measurements are likely,to be made at all cross sections (with the exception of using Manning's equation with no velocity calibration), thus the discharge could be calculated for any of the cross sections. The accuracy of discharge measurements is greatly influenced by the amount of variation in the cross section. Thus, if discharge were calculated for all the cross sections, one might find large discrepancies in the discharge. Therefore, the discharge should be calculated from the most uniform cross section in the study area. If all cross sections are highly non- uniform, it may be desirable to find one outside the study area exclu- sively for discharge measurement. The discharge should be calculated to at least two significant figures, or should be consistent with the allowable number of significant-figures based on the precision of the stream gaging measurements. Discharge measurement techniques are included in Appendix B. VELOCITY DISTRIBUTION If only one set of field measurements will be used with the Manning equation, velocity measurements should be taken with the calibration of Manning's n in mind. This means that the velocity measurements should coincide with the major breaks in topography or substrate observed during the measurement of the cross sections. These breaks usually serve as the boundaries to segmentation of a cross section into sub- sections. A mean velocity for each segment is required for calibration. If multiple sets of paired velocity-discharge measurements are to be used in the rating curve approach, measurements may be made at major breaks as above, or evenly spaced across the cross section. It is imperative that all subsequent measurements of velocity are made at the same locations as the initial measurements. Therefore, field notes should locate the positions of velocit measurement pots in terms of their distance from the dstake, not from water's edge. Failure to - follow this procedure may negate the use of this hydraulic prediction procedure, and has been a frequent source of error in the past. The location of each velocity measurement is called a "vertical." The number of verticals used per transect depends on the complexity of the velocity distribution and the detail desired by the researcher. The number of verticals used will usually be relatively independent of stream size. For the determination of the velocity distribution, 10-20 ?T