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normally be neglected in flood flow frequency analysis. Peak flow estimates of historic floods can be substantially in error because of the uncertainty in both stage and stage-discharge relationships. At times errors will be apparent or suspected. If substantial, the errors should be brought to the attention of the data collecting agency with supporting evidence and a request for a corrected value. A more complete discussion of sources of error in streamflow measurement is found in (7). V. Determination of Frequency Curve A. Series Selection Flood events can be analyzed using either annual or partial-duration series. The annual flood series is based on the maximum flood peak for each year. A partial-duration series is obtained by taking all flood peaks equal to or greater than a predefined base flood. If more than one flood per year must be considered, a partial- duration series may be appropriate. The base is selected to assure that all events of interest are evaluated including at least one event per time period. A major problem encountered when using a partial-duration series is to define flood events to ensure that all events are independent. It is common practice to establish an empirical basis for separating flood events. The basis for separation will depend upon the investigator and the intended use. No specific guidelines are recommended for defining flood events to be included in a partial series. A study (8) was made to determine if a consistent relationship existed between the annual and partial series which could be used to convert from the annual to the partial-duration series. Based on this study as summarized in Appendix 14, the Work Group recommends that the partial-duration series be developed from observed data. An alternative but less desirable solution is to convert from the annual to the partial- duration series. For this, the first choice is to use a conversion factor specifically developed for the hydrologic region in which the 8