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located in nonmountainous areas within about 60 miles of the study area. Al- though there is considerable scatter about the regression line it must be remem- bered that the observed Ql 0 0 for these stations is based on relativel y short periods of record (8 to 26 years; average about 16 years) and is consequently subject to considerable time-sampling error as previously discussed. The QIOO as indicated by the observed flood-frequency relation was less than the regional results for 11 of the 15 stations studied in this analysis. In general, then, figure 6 indicates an unbiased relation, and also provides a limited test of equa- tions 15 and 19 on additional basins in the study area and in adjoining States and river basins. This type of test could not be performed on the other peak- discharge relations or on the flood volume relation. Based on the drainage areas sampled, the regression equations presented are applicable to basins with effective drainage areas ranging in size from about 0.50 to 15.0 mi' that are in the plains region of the Arkansas River basin of Colorado. However. figure 6, along with table 9, indicates these results also may apply to basins as small as about 0.2 mi' or as large as about 30 mi', or located near the study area but within adjoining States or river basins. It is emphasized that the effective drainage area as used throughout this analysis excludes sub- basins above detention reservoirs or other manmade structures within the basin that would affect flood peaks. The effective drainage area must, therefore, be carefully determined from map? or aerial photographs showing such features. The data and analysis presented in this report have only dealt with rural basins with natural-flow characteristics. Although the effects of urbanization are not specifically addressed in this study, planners and designers are sometimes required to estimate the flood characteristics of small basins that have some urban development. Good agreement between the observed and estimated QIOO for the six small urban basins is indicated in figure 6 and table 9. Although urbaniza- tion within a basin generally results in larger peak discharges for a particular recurrence interval, it would appear these effects are relatively minor for QIOO' when effective impervious area is less than about 30 percent. Effective imper- vious areas are impervious areas which are connected and do not drain to pervious areas (Ellis and Alley, 1979). This analysis has focused on the characteristics of flood flows resulting from typical thunderstorm activity and the results may not apply to floods resulting from frontal-type storms. The user is cautioned that the results and equations discussed herein are preliminary and may be revised after analysis of new data. SUMMARY AND CONCLUSIONS A network of 17 stations provided an average of about 9 annual flood peaks per station and 141 rainfall-runoff events in a study to evaluate the magnitude and frequency of flood discharges and volumes for small rural basins in the Arkansas River basin of Colorado. From these recorded data, station flood- frequencies and the relation between peak discharge and flood volume were deter- mined. In addition, a rainfall-runoff model was calibrated for 15 of the water- sheds. and used to produce a long-term synthetic annual flood series. From these synthetic data, station flood-frequencies and the relation between peak discharge and flood volume were again determined. An improved estimate of the 29