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lJ1J062~ 4 Prairie Naturalist 24(1) March 1992 Platte between Lexington (U.S. Army Corps of Engineers rivet mile (rm) 251) and Chapman, NE (rm 157). The reach of the Central Platte between Chapman and the mouth of the Loup River was excluded because it has not been used for nesting for about 10 years. We examined the entire Lower Platte (rm 0-101). For some analyses, the Lower Platte was divided further into the reaches above and below the mouth of the Elkhom River (rm 33), a major tributary. METHODS Location of Nesting Sites During 13-24 June, we travelled by airboat from Lexington to the mouth of the Platte locating nesting sites and marking individual nests. We located addi- tional nests during foIlow-up visits to sites. We floated eggs to determine incu- bation stage (Hays and LeCroy 1971 as modified by Schwalbach 1988) and estimated the date of nest initiation by back-dating from the census date. We marked nests with survey flags placed 1 m north of the nest and pushed into the sand exposing 2 cm of roIled flag. Flags facilitated the relocation of nests later in the season for a variety of habitat measurements. Data collection while birds were present would have caused excessive disturbance. Aerial Videography We videotaped the Platte River from an aircraft on several dates to obtain areal habitat measurements during the breeding season (Sidle and Ziewitz 1990). FoIlowing the nesting season, we determined that the period 25 April-24 June had been the principal period of the birds' arrival and nest initiation. We selected six video recordings over the Lower Platte and eight over the Central Platte that represented the range of river discharges that occurred during this period because several of the habitat characteristics of interest to us varied with river discharge. We analyzed selected scenes from these recordings using image processing tools on a micro-computer (Sidle and Ziewitz 1990). We analyzed video scenes of 26 nesting sites to measure characteristics of used sites and analyzed a systematic sample of sites to measure the same char- acteristics of the riverine habitat available to least terns and piping plovers. We established samplc sites every three miles beginning at rm 2, which yielded 34 sites on the Lower Platte and 29 sites on the Central Platte. Three of the sites on the Lower Platte occurred at approximately the same location as nest sites. Therefore, the systematic sample of the Lower Platte was not entirely a sample of unused sites, and we included data from these three sites in both data groups. None of the Central Platte sample sites coincided with nest sites. The object of analysis within each video scene was a 402-m (X-mile) seg- ment of river. Within each segment, we measured the area of total channel, per- manent vegetation, and potentiaIly suitable nesting substrate (sandbar area). Such substrate appears white or light-colored on the video. Damp areas or areas with greater than 20% vegetation cover appear darker and were not interpreted as potentially suitable nesting substrate. We calculated average channel width as the active channel area divided by the 402-m length of the segment. We defined active channel as that portion of the river bed not stabilized by permanent "