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Table 2. Mean and standard deviation of number of days with <br />flows equalling or exceeding 10,000, 15,000 and 20,000 cfs during <br />the three sample periods of record. <br />>10,000cfs >15,000cfs >20,000cfs <br />1897-1965 21.$ (19.5) 7.1 (10.1) 2.2 (5.0) <br />1966-1993 5.7 (13.4) 1.3 (5.0) 0.2 (0.9) <br />The data in Table 2 indicate that the pre-dam period had, on <br />average, a period of 3 weeks each year with flows greater than <br />10,000 cfs. The post-dam has, on average, less than 1 week of <br />flows greater than 10,000 cfs. The pre-dam period also had, on <br />average, a period of approximately 1 week with flows greater than <br />15,000 cfs, while the post-dam period has one day per year with <br />these flows. The frequency of days with flows greater than <br />20,000 cfs is 10 times as great in the pre- vs. the post-dam <br />periods of record. We-were in the study area on 1 June 1993 when <br />the daily average flow was 15,300 cfs. Overbank flooding was <br />occurring on this day. We do not know the exact flows that cause <br />overbank flooding in the study area, but estimate that <br />approximately 12,000 to 15,000 cfs will cause overbank flooding. <br />Using Table 2 we estimate that overbank flooding would have <br />occurred, in the pre-dam period, an average of 7 days per year. <br />However, Figure 4 indicates that flooding would not have occurred <br />annually,, and when it did flooding may been sustained for 2 to 3 <br />weeks. The maximum flow for 1993 was 20,500 cfs on 18 May. <br />Years with one day with flows over 20,000 cfs occur in the post- <br />dam period once every 5 years. Thus, the 1993 flows were <br />extraordinary. Figure 2 indicates that total flows during the <br />1993 water year were the 4th largest since 1957. <br />8 <br />