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I I I I f J t t 1/ ~ I W l ~ \ I , "'." ~ ~ ~l r ,,:1' J:) t~ '.... ... SECTIO.FOUR leastTem therefore, productivity varies greatly from year to year. Populations could be maintained if fledging success is high periodically (Mertz 1971; Caswell 1982). 4.4.2 Importance of the Platte River Considerable population data has been collected on least terns along the central Platte River. Habitat utilized in the central Platte River ecosystem includes the riverine habitat associated with the river sand and gravel spoils areas associated with past and ongoing sand and gravel dredging operations. River Channel Habitat Features Location and total number of adult least terns on sandbars on the central Plane River during the II-year period (1988 through 1998) are provided in Table 4-2. Mapped locations of riverine and sand pit nesting areas within the central Platte River are provided in Appendix A. Field research in 1988 on least terns along the Platte River censused and examined least tern riverine habitat using ground transects and aerial videography (Ziewitz et aI. 1992). During 1988, a total of 222 least tern nests at 30 sites was located on the Plane River. Most nesting occurred on the lower Platte River. Least tern nest initiation dates ranged from May 19 to June 23. The median nest initiation dates for least terns was June 7. They nested earlier on the lower Platte River than on the central Platte River. In the aerial videography of the central Platte River, moderately vegetated sandbars and sandbars only slightly exposed above water (not suitable nesting habitat) were common (ZiewitZ et al. 1992). Areas that could clearly be interpreted as potentially suitable nesting substrate were rare. River discharge at Grand Island ranged from 25 to 2,570 cfs on the eight sampling dates of aerial videography. With one exception, sandbar areas greater than 2 acres were measured only on the two lowest flow dates (June 15 and 24). The mean sandbar area of nest sites was 1.2 acres for the eight dates examined. Ibis was significantly greater than the mean of 0.6 acre for systematic sample sites. The mean channel width of nest sites was 975 feet, significantly greater than the mean of 660 feet for the systematic sample sites. Most nests (90.3 percent) had less than 50 percent vegetative cover in the surrounding 3-foot square area, and all had average vegetation heights less than 2 feet (Ziewitz et aI. 1992). On the central Platte River, the mean elevation above the 400 cfs stage at nest sites ranged from 0.2 to 2.0 feet, with a mean of 1.0 foot. The mean elevation at the systematic sample of sites ranged from 0.1 to 1.8 feet, with a mean of 0.4 foot. These means were not significantly different. The maximum elevation above the 400 cfs stage at nest sites ranged from 0.4 to 4.4 feet, with a mean of 2.7 feet. The maximum elevation at the systematic sample of sites ranged from 0.3 to 3.7 feet, with a mean of 1.4 feet. These means were not significantly different (Ziewitz et aI. 1992). In summary, comparisons of sandbar area, channel width, mean elevation, and maximum elevation of nest sites versus systematic sample sites in the Platte River suggest that least terns use wide channels with a large area of dry, sparsely vegetated sand. By these two measures alone, habitat availability was considerably greater on the lower Platte River than on the central fIllS GnliDer ~ant "'* FetIetII Senfl:es 56F0097286OOh1.ox. 6I'2l"~.9".'52. NA)I\)'R.SGV\'tfS/2 4-5