Threats to Wildlife and the Platte River

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WILDLTE p. 35 The authors conclude that "Changes in hydrology could affect the quantity of sand supplied to the Platte. A reduction in the frequency of effective flows will reduce the sediment transport capability of the river ... Changes in sediment transport of the South Platte River could sig- nificantly impact sediment loads of the Platte River" (Lyons and Randle 1988, p 26). G The often -cited flow rate of 3,800 cfs has an interesting history. The number first appeared in Karlinger et al.'s (1981) highly technical theoretical paper as an example of how field data on channel slope and bed material size could be used in conjunction with Parker's (1978) equations to calculate what flows are necessary to maintain a given width of channel in a particular segment of river. "For a hypothetical example, a minimum channel width of 500 ft. for a reach of the Platte River near Overton, Nebraska, is required for roosting of sandhill cranes [The width of the channel is not hypothetical; Sandhill Cranes do prefer this width, and this is also the lower limit to channel width used by Whooping Cranes (Krapu, et al. 1984, Currier and Eisel 1984)]. Wildlife managers would like to know the discharge necessary to maintain this width of channel for the given reach, and if possible, what duration and frequency of this discharge will give optimal results" (p 13). This calculated flow rate for the stretch of river near Overton was 3,800 cfs. Boudreaux then generalized this flow rate ("For simplicity and practicality, I recommend that one effective discharge be used for the entire reach between Overton and Grand Island" (1982, p 9)), and his generalizations were used by the Fish and Wildlife Service and Nebraska Game and Parks Commission, and cited by the Bureau of Reclamation in various biological assessments and opinions (see next footnote). Support for this widely used flow rate has begun to dry up among biologists and hydrologists. At an ad -hoc hydrology committee meeting convened in Lincoln, Nebraska, by the Fish and Wildlife Service on November 10, 1986, "discussion was initiated with an analysis of the 3,800 cfs channel maintenance flows. This value was promptly dismissed without prejudice" (O'Brien 1986). H The flow rate is calculated as that which would maintain a 500 -foot wide channel in the reach near Overton. The 16 -day duration is calculated as that which, at a flow rate of 3,800 cfs, would provide streampower equivalent to the amount which had "been instrumental in maintaining the channel' during the May- through- August seed - germination period during the preceding 28 years. Equal spacing of these 16 days was suggested for the inhibition of both early and late - falling seeds. I Karlinger et al.'s (1981) example flow rate of 3,800 cfs as that which would maintain a 500 ft. wide channel near Overton was used by Boudreaux, a water resource engineer at a private consulting firm (see footnote 7"). Boudreaux also used a personal communication with G. Miller of the FWS' Grand Island office wherein Miller suggested that the seed - germination period (May- August) may not be as critical as once thought. Boudreaux then calculated in the following manner that scouring could be accomplished during any 23 days per year: He assumed flows larger than 3,800 cfs would not be permitted. He then analyzed annual flow duration curves since 1950 ("during which channel changes were not rapid; i.e., channel was quasi - stable... at Overton "), and found that flows of 3,800 cfs were equalled or exceeded an average of 14 days per year. [I]n order to get the same... streampower... the 3800 cfs ... (as- suming larger flows are not permitted) should be maintained for 23 days per year."