was a time when any existing nests would have been flooded. An eight-week duration was used as the interval of time typically required for successful reproduction of the birds (fledging). As stated above, the falling limb of the June rise below 7-foot stage is a potential interval for successful reproduction if that interval is 8 weeks in length and does not extend beyond September 1 and if no significant short term rises occurred. (Note: the summer low flow proposed in the 2002 biological opinion has no particular advantage for successful reproduction as compared to present flow management practiced the Corps). These criteria were used to evaluate the ‘natural’ hydrograph for the years 1929 – 1955. The analysis, which is summarized in table 1, indicated that for 24 of the 27 years successful reproduction was likely trivial or non-existent because the sandbars would have been expected to have been completely flooded during the ‘heart’ of the tern and plover reproduction season. The sandbars were not completely flooded in 1934; however, on June 16 there was a 2.8-foot water level rise. It is likely that this rise would have flooded most of the tern and plover nests. (This analysis does not consider potential of nesting on sandbar banks of the River. These sites are very susceptible to damage by predators. Nor does this evaluation consider nesting off river that occurs in cutoff chutes or adjacent cutoff lakes.)
The ‘Offset, Days” column of table 1 shows that typically the initial date that the river water temperature reached 18 degrees Celsius for seven consecutive days occurred after the date of the starting of the June rise. However, for some years the date occurred before the start of the spring rise. There was a large variability from –52 days to plus 11 days. The large variability suggests that the two occurrences are largely coincidental and not ‘coupled’ per se. This is expected because the June rise is the result of melting snow pack in the Rocky Mountains whereas the water temperature is largely the function of the mean-daily temperature of the date and the three preceding days. Accordingly, the assumption that the pallid sturgeons are cued by a coupled flow and temperature cue needs careful evaluation and further study.
The comparison of stage data for the natural hydrograph for the years 1929-1955, suggests that the hydrograph was very unfavorable to the successful reproduction of the least terns and piping plover on the island sandbars of the Missouri River. It is possible that when the birds arrived from late April through early June that some may have tried nesting. However, because a June rise greater than 7 feet, which would have likely completely submerged all the sandbar islands, occurred 28 of the 29 years investigated, the ‘early’ nests would have been flooded. The birds are opportunistic nesters and they could have been expected to search out other nesting sites off the river. If the migrating birds found the sandbar islands flooded in the spring, it is likely that they would have continued their migration in search of more suitable nesting sites. The period of investigation was from 1929 through 1955 and included the drought year of the 1930s. The average rank for each year is shown in the rank column. The average rank for 1929-1955 record is 66, whereas the average rank for the 1898-1998 historical record is 50.5. Thus, the 1929-1955 record is more dry than normal. Accordingly, the analysis of the