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<br />of uncertainty in the relation; that is, a relation based upon nonrandom <br />data will have a degree of reliability attainable from a lesser sample <br />of random data (5), (6). <br /> <br />C. Watershed Changes <br />It is becoming increasingly difficult to find watersheds in which <br />the flow regime has not been altered by man's activity. Man's activities <br />which can change flow conditions include urbanization, channelization, <br />levees, the construction of reservoirs, diversions, and alteration of <br />cover conditions. <br />Watershed history and flood records should be carefully examined to <br />assure that no major watershed changes have occurred during the period of <br />record. Documents which accompany flood records often 1 ist such changes. <br />All watershed changes which affect record homogeneity, however, might <br />not be listed; unlisted, for instance, might be the effects of urbaniza- <br />tion and the construction of numerous small reservoirs over a period of <br />several years. Such incremental changes may not significantly alter the <br />flow regime from year to year but the cumulative effect can after several <br />years. <br />Special effort should be made to identify those records which are <br />not homogeneous. Only records which represent relatively constant <br />watershed conditions should be used for frequency analysis. <br /> <br />D. Mixed Populations <br />At some locations flooding is created by different types of events. <br />For example, flooding in some watersheds is created by snowmelt, rainstorms, <br />or by combinations of both snowmelt and rainstorms. Such a record may <br />not be homogeneous and may require special treatment. <br /> <br />E. Reliability of Flow Estimates <br />Errors exist in streamflow records, as in all other measured <br />values. Errors in flow estimates are generally greatest during maximum <br />flood flows. Measurement errors are usually random, and the variance <br />introduced is usually small in comparison to the year-to-year variance <br />in flood flows. The effects of measurement errors, therefore, may <br /> <br />7 <br />