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frequency curve is about the same as for the upper basin western tributaries (station 07083000 on fig. 95), which is less than the slopes of the other plots on figure 95, except for station 07096000. Unlike station 07083000, however, the seasonal distribution of monthly runoff (fig. 96) for station 07119500 does not account for much of the variation. The spring and summer months have higher mean runoff, demonstrating the effects of the thunderstorm runoff and irrigation-return flow. No snowmelt peak or recession is seen. Minimum runoff in every month is about the same. The band showing one stand- ard deviation about the mean goes to the bottom of the graph for much of the year because the monthly standard deviation is greater than the mean monthly runoff for those months. Seve~al otner tributaries ente~ the ~rkansas River along its course on the Great Plains. The headwaters of these tributaries are in the prairie, and, in general, very little runoff Occurs whether they have large or small drainage areas. Much of the runoff is a result of irrigation-return flow and waste water, and these drainages only contribute their own runoff after heavy precipitation events. Station 07134100 (Big Sandy Creek near Lamar) is the largest of these tributaries, but it was selected as representative of all of these smaller prairie tributaries. The runoff for this site is the lowest of the six stations discussed (fig. 95). The probability of the monthly runoff being less than or equal to 0.000020 in. is 1 percent, and the probability of monthly runoff being greater than or equal to 0.020 in. is 1 percent. The monthly distribution of runoff (fig. 96) also shows none of the characteris- tics of any of the other stations. The main stem of the Arkansas River downstream from Pueblo is a regulated stream affected by reservoir storage, irrigation diversions, and return flows. Although it is difficult to compare runoff from station to station, because streamflow decreases as drainage area increases, the characteristics of sta- tion 07133000 (Arkansas River at Lamar) are typical of those on the main-stem river downstream from Pueblo. Runoff for station 07133000 is highly variable (fig. 95), with a I-percent probability of the runoff heing less than or equal to 0.000027 in. (0.47 ft)/s) and a I-percent probability that the runoff will equal or exceed 0.66 in. (11,500 ft)/s). The monthly distribution of runoff (fig. 96) shows that, in every month, a large range occurs from minimum to maximum, and the standard deviation is greater than the mean monthly runoff. Correlations of monthly mean discharge and annual mean discharge were computed for every combination of pairs of stations. Because discharge results from an areal distribution of precipitation or snowmelt, and because many of the stations are downstream from one another on the same stream, dis- charges at different stations tend to correlate very well. The best intersta- tion correlation coefficient for the monthly means was 0.997. Of the stations that had concurrent record, 9.0 percent of those correlations was greater than 0.95; another 7.4 percent was greater than 0.90; and an additional 8.7 percent was greater than 0.80. The best correlation coefficient for the annual mean discharge was 0.999. Of the stations that had concurrent record, 5.0 percent of those correlation coefficients was greater than 0.95; another 6.6 percent was greater than 0.90; and an additional 15.7 percent was greater than 0.80. Selected classes of interstation correlation of monthly mean discharge for every pair of stations is presented in the IISupplemental Information" section at the end of the report. 65