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

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Messrs. Lochhead, Simpson, and Robotham 5 The Service's information shows that 62 percent (13 of 21) of confirmed - 0° "5 °w `c spring sightings on the Platte River have occurred at flows above 1,800 cfs. Furthermore, the Nebraska Game and Parks Commission (1993) found that 64 percent of confirmed whooping crane roost nights (spring and fall combined) were at flows greater than 2,000 cfs, and 39 percent greater than 2,400 cfs. Some experts believe length of stay is associated with habitat quality. In this regard, of the six times when stays of multiple nights where documented, five had average flows greater than 2,000 cfs (Nebraska Game and Parks Commission 1993). The targets prepared by the Service would provide a range from 2,400 cfs during 54 percent of whooping crane migrational periods to lesser amounts of 1,800 cfs and 1,300 cfs in about 46 percent of the periods. Page 6. 2.1. Peak Flows Comment: Colorado asks for clarification of "wet" and "very wet" frequencies of peak flows. "What is the basis for the exceedence frequency for peak flows? . it is unclear from the Service's analysis whether these frequencies are based on the hydrologic record, or whether they are based on biological criteria." Tables 2 and 3 in Bowman and Carlson (1994).list the recommended exceedence probability of the peak flows in the right -most column (i.e., the value in parentheses). Although historic flow information was part of the information base used to quantify the frequency of the recommended flows, they are based primarily on scientific criteria to maintain channel width. The first column in tables 2 and 3 lists the terms "very wet," "wet," "normal," and "dry," and these were intended to be a general description of the frequency of the target pulse flows. These terms should not be given much weight beyond that general label. The exceedence probability in the right -hand column of those tables identifies the desired probability of occurrence for the pulse flow targets and not the general labels of "very wet," etc. For example, the target exceedence probability of a 12,000 -cfs event is 40 percent. This means that, in any given year, the recommended probability of one or more events of 12,000 cfs or higher occurring is 40 percent. Comment: How often have desired flows occurred historically during the. desired timeframe, and how does this historic record relate to the recommendation that peak flows occur more than 50 percent of the years during the May 20-June 20 timeframe? A great deal of confusion and misunderstanding has resulted from characterizations of the Service's flow recommendations and flow targets as being for the purpose of "maximizing" or "optimizing" habitat conditions. Accordingly, the Service does not refer to its target flows as "desired flows." When the Service revised its flow recommendations in March and May 1994, the context of discussions was the minimum magnitude, frequency, duration, and quality of instream flows at which existing habitat conditions could be maintained and degraded habitats rehabilitated.