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drought or excessive floodings, the interruption should not be ignored but rather should be used, if possible, as evidence for adding one or more estimated peaks to the systematic record. Thus, the systematic record is intended to constitute an unbiased and representative sample of the population of all possible annual peaks at the site. In contrast to the systematic record, the historic record consists of annual peaks that would not have been observed except for evidence indicating their unusual magnitude. Flood information acquired from old newspaper articles, letters, personal recollections, and other historical sources almost invariably refer to floods of noteworthy, and hence, extraordinary size. Similarly, the very existence of an indirect streamflow determination outside a period of systematic record suggests that the detennination was made because an unusually large streamflow had occurred. Thus, historic records, by the conditions of their collection, form a biased and unrepresentative sample of flood experience. Despite this bias, however, the historic record can be used to supplement the systematic record provided that all historic peaks above some historic threshold have been recorded. The systematic record may contain one or more peaks for which historic information is available or which exceed the smaller historic peaks. Such peaks are called high outliers. They are used as part of the systematic record but also are treated like historic peaks in the historic-record adjustment procedure. The U.S. Geological Survey maintains a peak flow data base. In the past, these data were stored in the WATSTORE peak flow file. The data are now maintained in the National Water Information System data base. Qualification codes may be assigned to some peaks, identifying (I) basin or environmental conditions that may have affected the magnitude or accuracy of the streamflow value or (2) historical peaks. For example, if the peak is too small to measure, then an estimate of lower bound on the magnitude may be stored (this might occur during a drought period when the actual peak was less than the gage could record). In this case, the peak-flow file would contain a qualification code of 4 (Lepkin and others, 1979) associated with the peak. Note that an individual peak flow can have more than one qualification code associated with it. Table I contains a description of the qualification codes that can be found in the peak-flow file. To make things interesting, peakfq recognizes a subset of peak-flow-file qualification codes and uses a different code to identify these peaks. Table la defines the correspondence between the peakfq and peak-flow-file qualification codes and briefly describes how the peakfq program handles the associated peaks. Table 1. Streamflow-qualification codes for peak streamflow Streamflow qualification code Definition 1 2 3 4 5 6 7 8 9 A B C Streamflow is a maximum daily average. Streamflow is an estimate. Streamfiow affected by dam failure. Streamflow less than indicated value, which is minimum recordable value at this site. Streamflow affected to an unknown degree by regulation or diversion Streamflow affected by regulation. At least 10 percent of basin controlled by reservoirs. Streamflow is an historic peak. Streamflow actually greater than indicated value, Streamflow due to snow melt, hurricane, ice-jam or debris dam breakup. Year of occurrence is unknown or not exact. Month or day of occurrence is unknown or not exact. All or part of the record affected by urbanization, mining, agricultural changes, channelization, or other activity, The urbanized basins contain at least 10 percent impervious cover. Only peak streamflow recorded for this year. E PEAKFO 2 DRAFT - 1/30/98