Nesting of the least terns and piping plovers on the rising limb of the June rise would seldom be successful because the islands are typically completely covered by the spring rise floods. All ‘early’ nests would be inundated and nesting made impossible. Because of the length of flooding, there seldom is adequate time remaining for late nesting on the falling limb of the June rise. In addition, any significant short-term rises on the falling limb may flood the late nests. It is not necessary that the sandbars be completely flooded form June through July to negate the birds nesting efforts. If a flood peak reaches 7 feet once or more during that period successful nesting is unlikely.
The range of heights of both the least tern and piping plover nests above the water level may be from a few inches to about 3 feet. Schwalbach (1988) concluded that piping plover nests were about 12 inches above water level. This seemingly low height was likely the result of the birds being ‘forced’ to nest that low because of lack of suitable nesting habitat. The range of heights of the least tern and piping plover nests above the river level is from a few inches to several feet. The heights are seemingly determined by width and slope of the suitable nesting habitat and by the distance from water.
Beacom (2003), based on a review of historic observations of the nesting of the least tern and piping plover, concluded it would be unlikely that satisfactory sites for successful nesting for the birds would be available because of spring rise flooding. Stiles (1955, p. 19-21) stated that “it is usually about the first of August before the river has receded sufficiently to expose sandbars and islands which form the nesting grounds” for the least tern. In 1937, the sandbars in Sioux City were covered at a gage height of 7.2 feet (Hardy, 1957). Information shown in figure 9 is supportive of the information presented by Hardy because there is a change of the discharge-stage relationship at 7 feet. Beacom (2003, p. 7) stated “this observation and a close study of pre-impoundment river stages and flows… clearly indicates that the least tern and the piping plover would rarely find habitat available on the Missouri river in May when they migrated north.” For example, figure 11 shows that flooding greater than 7 feet occurred in 1929 from about May 11 through July 21. It is likely that the terns and plover did not nest on the sandbars above Sioux City in 1929.
The stage and temperature hydrographs, such as figure 11 and those in Appendix B, can be used to evaluate flooding of sandbars and the relation of water temperature to stage. The following criteria were used to graphically evaluate the hydrographs. In reference to ‘coupling’ of temperature and the June rise, the first date that the water temperature reached 18o C for one week was compared to the date that the June rise started. In reference to evaluating natural hydrograph for potential for successful reproduction of the terns and plovers, the first date that a June rise river stage was 7- feet or more was noted, and the last date during the summer that the stage was at least 7-feet was noted. This interval was considered as an interval when nesting either could not occur as the bars were flooded, or it