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~ . 3 . Results The results of this analysis are shown in Table 1. Typically, bankfull flow occurs once every two years wIth an average duration of 10-15 days. As discussed previously, the Pgm for Cleopatra's Couch is calculated from mean daily flow records. Given the distance downstream from gages 9-2510 and 9-2600, the instantaneous peak flow at Cleopatra's Couch is attentuated relative to the peak at either gage. Thus, the estimate calcu- lated from mean daily flow approximates the instantaneous peak flow at Cleopatra's Couch. Channel conditions at Cleopatra's Couch have been intensively studied by O'Brien (1964\. He estimated Pb to be 21,500 cfs for a cross-section near the Cleopatra's Couch site by measuring channel shape and discharge. This exceeds the calculated pgm of 13,100 cfs and is capable of moving the median bed particle at the measured cross-section. This difference is likely related to physiographic setting; the steep canyon walls and side canyons contribute large amounts of coarse material to the Yampa River channel. This is suggested by the high flow necessary to entrain the median bed particle. Thus, O'Brien's estimate of 21,500 cfs necessary to mobilize the median bed material represents a channel-modifying flow for the lDwer Yampa River but has a return interval in excess of 20 years. The approxi- mation of formative discharge by the Pgm is best suited for channels formed in alluvium. This is demonstrated by comparing the Pgm of 13,100 cfs for Cleopatra's Couch to the effective discharge of 11,500 cfs calculated from sediment data (washload) collected upstream (O'Brien, 1984). Discharges in this range contribute the greatest p~rtion of the total sediment load for the Yampa River but do not entrain much of the bed material at the Cleopatra's Couch site. Yet, the lower Yampa River Is influenced by a wide range of flows because of it's canyon setting; /