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<br />UUU6J6 <br /> <br />16 <br /> <br />Prairie Naturalist 24(1): March 1992 <br /> <br />Channel Dimensions <br />Habitat restoration in the Central Platte should be undertaken in the widest <br />channels. Mean channel width of nest sites on the Central Platte was 295 m. <br />Mean channel width of the 402-m (:I.-mile) reaches immediately above and be- <br />low the Central Platte nest sites was 275 m. Channel width was calculated as the <br />active channel area divided by 402 m. Active channel area is the total channel <br />area minus the area of permanent islands. We recommend undertaking habitat <br />restoration in approximately 1200-m reaches where channel width 'is at least <br />295 m in the central 400 m and at least 275 m in the upper and lower 400 m. As <br />in our sampling scheme for study sites, these dimensions apply to a single <br />contiguous channel, not to multiple channels on opposite sides of long, <br />permanent islands. <br />If habitat restoration proves successful in channels that meet the width and <br />length requirements described above, similar restoration projects should be un- <br />dertaken in narrower channels, because more narrow channels exist in the Cen- <br />tral Platte. We recommend further habitat restoration in 1200-m reaches where <br />channel width is at least 181 m, the width of the narrowest site used for nesting <br />in 1988. Clearing vegetation from permanent islands, which does not in itself <br />increase active channel area or channel width but does increase visibility at a <br />site, is a third option that may provide conditions suitable for a nesting sandbar. <br /> <br />Sandbar Area <br />Within the central 400 m of a suitable 1200-m reach, one or more mid-chan- <br />nel sandbars should be dredged to provide a high, relatively clean, and smooth <br />nesting substrate. Nest sites on the Lower Platte had a mean of 1.45 ha of dry, <br />sparsely vegetated sand during the period when many nests were initiated (data <br />from 24 May aerial videography, Table 1). <br />USFWS has identified 22.7 cms (800 cfs) during the summer in the Central <br />Platte as a flow to maintain forage fish for least terns and to restrict sandbar <br />access by mammalian predators and recreational vehicles (USFWS 1987a, <br />1987b; Sidle et al. 1990). Using the Lower Platte as a nearby model of better <br />habitat conditions, we recommend that the size of the restored sandbars on the <br />Central Platte should be at least 1.45 ha relative to a discharge of 22.7 cms. <br />Given the apparent preference shown for greater sandbar area in the comparison <br />of nest sites with systematic sample sites, sandbars larger than 1.45 ha should be <br />created wherever practical. Sandbars of up to 4.04 ha located in sufficiently <br />wide channels with adequate nesting season flows should provide suitable nest <br />sites. <br /> <br />Sandbar Height <br />At the Grand Island river gage, a discharge of 22.7 cms is equivalent to 0.45 <br />m above the stage of 0.0 cms (for simplicity, the stage of 0.0 cms shall be con- <br />sidered the lowest portion on a cross section of the river bed). At least 1.45 ha <br />of the restored sandbar, which is the minimum size described above, should be <br />greater than 0.45 m in height. The mean peak discharge for 15 June to 31 <br />August during 1957-88 was 149 cms (5250 cfs). which is equivalent to a stage <br />of 0.91 m. A sandbar of this height would have escaped inundation between 15 <br />