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are not bordered by water on all sides. Studies by other researchers (e.g., <br />Schulenberg and Schulenberg 1982) have found that flooding, which primarily <br />affects colonies on river sandbars as opposed to off -river sandpits, can <br />actually increase the incidence of predation. As rising water levels cause <br />adult birds to abandon their nests, the unprotected eggs and chicks become more <br />vulnerable to predation. <br />HABITAT USE CONSIDERATIONS RELATIVE TO THE CENTRAL PLATTE RIVER <br />Three lines of evidence support the conclusion that the central Platte River <br />from Overton to Grand Island, Nebraska (commonly referred to as the Big Bend <br />reach), did not play a significant role in the maintenance of least tern or <br />piping plover populations prior to the activation of upstream water projects in <br />1941 (i.e., Kingsley Dam and the Tri County Canal system). First, peak flows <br />historically occurred during the species' nest selection and /or nest initiation <br />period (i.e., early May -early June). Given the original pattern of high flows <br />from mountain snowmelt commencing in late April or early May and peaking in <br />June, it follows that during most years potentially suitable sandbar habitat <br />would have been largely inundated at the time of arrival and nest selection. <br />Nests initiated before the period of peak flow, in years of delayed snowpack <br />thaw, would have been subject to flooding. Second, extended no -flow episodes <br />occurred during the nesting season. The historical tendency of the central <br />Platte reach to go dry during summer months was not conducive to successful <br />nesting. Dry riverbed conditions would have eliminated adjacent feeding areas <br />and exposed nest sites to predation. Third, despite regular observations of <br />least terns and piping plovers in Nebraska since the turn of the century, <br />historical sighting records contain no reference to the occurrence of these <br />species along the Platte River from east of the confluence of the North and <br />South Platte rivers to the Loup fork until after 1941. It therefore seems more <br />than coincidental that the first record of least terns nesting along the <br />central Platte was in 1942 near Lexington- -the very year perennial flow was <br />established in this reach by water project operation - -and that the first report <br />of piping plovers in this same area was in 1950. <br />These considerations instead indicate that regular use of the Platte River <br />above the Loup confluence (and including the Big Bend reach) by least terns and <br />piping plovers is a recent phenomenon occasioned by the onset of permanent, <br />moderate flow conditions resulting from water project operation. Water storage <br />along the Platte has reduced dramatic, natural water -level changes from floods <br />or droughts that historically acted to limit utilization and /or nesting success <br />through inundation of potential or active nest sites, elimination of adjacent <br />food supplies, and exposure of nest sites to predation. Paradoxically, the <br />progressive shift to perennial flow that created suitable nesting habitat for <br />these species has sustained the growth and perpetuation of woody vegetation. <br />Optimum conditions (in terms of the maximum quantity and quality of suitable <br />riverine habitat) should therefore be viewed as transitory, though the future <br />situation appears more favorable than the past. In addition, sand and gravel <br />operations near the river have also created substantial additional nesting <br />habitat which has been used extensively by these species. As previously noted, <br />these sites are.particularly important during years when high river flows limit <br />or preclude the use of mid - stream sandbars (e.g., 1983 and 1984). Because <br />sandpit colonies are less susceptible to flooding (a dominant source of <br />mortality at sandbar colonies), they have significant potential for successful <br />production, especially if active steps can be taken to control human <br />ix <br />