Threats to Wildlife and the Platte River

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p. 34 WILDLIFE D However, "there is evidence to indicate that this is not the case on the Platte... The river has responded to this reduction in sediment supply by degrading most of the channel in the Central Platte" (O'Brien and Currier 1987, p 39). "It was generally acknowledged that without the sediment load from the South Platte, the critical habitat reach is finished" (O'Brien 1986, p 3). Nonetheless, "the Platte may never completely transform into a meandering stream because its bed and banks contain insufficient percentages of silts and clays that are necessary to stabilize the channel and banks" (O'Brien and Currier 1987, p 40). E Interestingly, "pre- reservoir (prior to 1909) peak discharges exceeded all the threshold criteria on nearly an annual basis ... Since 1957, following the construction of Glendo, the last major reservoir in the Platte River system, the mean peak discharge has been less than 8,000 cfs at Overton. Clearly, the present channel - forming discharge is far less than the threshold to maintain a braided stream and the Platte is tending toward a meandering form" (O'Brien and Currier 1987, p 39). The river appears still to be adjusting to the flow changes of the last century. "[L]ong term trends of degradation and sediment size changes indicate the system was in disequalibrium [sic]...[We] are assuming the river system is still in disequalibrium ... There is vegetative growth continuing to be established in the river channel..." (USFWS 1987a). "[S]ites... upstream of...Overton are maintaining relative stability, while those sites downstream of Overton still are adjusting to the changes in the hydrologic system upstream... The Platte River near Odessa, Grand Island, and Duncan, Nebraska, still is adjusting toward stability" (Kircher and Karlinger 1983, p B -26). "None of the information, data, and analyses available to date have enabled any individuals to conclude, with certainty, that the Platte River as it exists today in Central Nebraska is in a state of equilibrium. If it is not in a state of equilibrium, the existing braided channel may trend towards a meandering form in the absence of any additional alteration of flow and related sediment load" (U.S. Bureau of Reclamation 1987, pp 111 -117). F In discussing the question of how much water is necessary to move sediment on the Platte to maintain a "braided river," Lyons and Randle do not suggest a specific flow rate, saying that "the concept of specific channel- forming discharge seems inappropriate ... the use of regime equations to describe the sediment, width interactions appear inappropriate at this juncture" (p 11). Lyons and Randle, however, do discuss the relationship of Platte River channel characteristics to flows in the 1,000 to 10,000 cfs range, comparing the periods 1926 -1939 and 1958 -1986. "Although the flow conditions in this range for these two time periods are similar, the current morphology of the Platte is much different than ... in 1938" (p 28). Comparing three periods, they note "The frequency of flows between 1,000 and 10,000 ft3 /s was less during the 1940 -57 period than—during the 1926 -39 period. Channel narrowing occurred at the same time the frequency of these flows decreased. However, the 1958 -86 period experienced an increase in ... flows in the 1,000 to 10,000 ft3 /s range, with no corre- sponding increase in channel width" (Lyons and Randle 1988, p 25). Lyons and Randle (1988) note that "present -day sand loads at Overton are about 30 percent of estimated historical values. Historically, the North Platte sand load [now mostly trapped behind upstream dams such as Kingsley] was at least 60 percent of the Platte's sand load at Overton" (p 10). Presently the sand load carried by the Platte is approximately equal to that carried by the South Platte (Lyons and Randle 1988, p 25).