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At the State line gage on the Colorado River, the mean peak flow is now 52 <br />percent of its historic mean. The greatest loss of natural flows occurs <br />during the months of May and June (Table 4). Fortunately, the frequency of <br />years when the peak flow at the State line gage reaches the level at which <br />maximum squawfish production has been recorded (30,000-40,000 cfs) has not <br />changed significantly; as during historic times, peak flows within this <br />'window' still occur in about one of every four years. What has changed is <br />the loss of the frequent high-volume years, which are important in keeping <br />the abundance of some of the non-native species in check and in maintain- <br />ing channel complexity. Prior to 1937, the annual peak flow at the State <br />line (16 years of record) exceeded 30,000 cfs 81% of the time, and exceed- <br />ed 40,000 cfs 56% of the time; in recent years (after 1965), peak flows <br />exceeded 30,000 cfs 33% of the time, and exceeded 40,000 cfs only 8% <br />percent of the time (Fig. 7). During the 16 years of historic record, peak <br />flows at the State line were never less than 26,000 cfs (Fig. 8). During <br />60 <br />HISTORIC 1908-23 <br />50 <br />40 <br />U <br />LU 30 <br />Z <br />d <br />UJ 20 <br />cc <br />U. <br />0 <br />COLORADO RIVER <br />CO-UT STATE LINE <br />10 <br />> 40,000 30-40,000 20-30,000 420,000 <br />CUBIC FT/SEC <br />Figure 7. Recent and historic frequencies of peak flows (highest flow <br />day of the year) of various magnitudes. Data from Appendix Table I. <br />30