Platte River Channel Becomes a Focus for Platte River Cooperative Program Studies2001

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Messrs. Lochhead, Simpson, and Robotham Information regarding the historic timing of peak flows is enclosed. The table entitled "Overton Annual Instantaneous Peak Flow" lists the historic peaks which occurred for the available period of record near Overton. This gage represents the longest -term record available for the critical habitat reach. The table shows that for the 1902 -1939 period (e.g., prior to operation of Seminoe, Alcova, Glendo, Grayrocks, and Lake McConaughy) that approximately 50 percent of the annual peaks occurred between May 20 and June 20. The post 1943 period, which is more representative of present conditions, shows 22 percent of the annual peaks occurred during the May 20 -June 20 period. 0 Please note that the percentages cited above were determined from the annual instantaneous peak data and, therefore, considers only the largest flow event in each year. Most years of the early flow records show that a dominant peak occurs during some broader time period of May -June. Though this aspect of the pattern of timing and frequency was not described in the previously recommended targets, the Service believes this pattern is a feature of the late spring pulse which should be emulated to the extent practicable. Comment: . . the Service's peak flow recommendations conform to the recommendations of only one of the experts who attended the workshop, Dr. James O'Brien. If true, why was Dr. O'Brien's information more compelling than that of the other gathered experts ?" The experts who attended the May 1994 workshop and discussed aspects of channel maintenance included Dr. Paul Currier, Dr. Carter Johnson, Mr. Joe Lyons, Dr. Jim O'Brien, Mr. Tim Randle, and Dr. Bob Simons. Your question concerns the flow peak (i.e., daily pattern) within the prescribed pulse. Though some of these experts discussed the peak (daily pattern) qualitatively, only one expert, Dr. O'Brien, provided specific recommendations regarding the desired duration, magnitude, and frequency. The Service's recommended targets —� included Dr. O'Brien's recommendations because they are compatible with the available information concerning changes in channel width, sinuosity, bed material particle size distribution, bed elevation, bed material transport, and effective discharge (Butler 1994). Much of that available information was developed by the experts listed earlier. What are the bases for his recommendations? Dr. O'Brien listed five methods he used to estimate the range of flows necessary to maintain the current braided segments of the Platte River. These included: Mean peak flow for recent conditions 7,300 - 8,100 cfs Bankfull Discharge 12,000 - 16,000 cfs Alluvial Return Period 7,000 - 7,800 cfs Scouring of Vegetation 8,000 - 12,000 cfs Incipient Motion 7.300 - 10,100 cfs Average 8,300 - 10,800 cfs