spawning. In reference to the proposed summer drawdown, lowering the river levels from June through August will tend to disconnect the river from the few backwaters in that reach.
Analysis of the hydrographs indicated that the June rise and the occurrence of a specific water temperature (approximately 18o C for the pallid sturgeon spawning) were highly variable and ranged from plus 11 days to minus 52 days. The temperature data suggest that the assumption that flow and water temperature were coupled in the natural hydrograph and cued the fish to spawn needs more investigation and careful reconsideration. This conclusion is consistent with other current studies that indicate that temperature is the important exogenous cue for pallid sturgeon spawning.
The comparison of stage data for the ‘natural’ hydrograph for the years 1929- 1955 indicates the natural hydrograph was very unfavorable to the successful reproduction of the least terns and piping plover on the island sandbars of the Missouri River largely due to complete flooding of the islands during the ‘heart’ of the reproductive season. It is possible that when the birds arrived, typically in 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 26 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 or to continue migration. If the spring migrating birds found the sandbar islands flooded, it is likely that they would have continued their migration in search of more suitable nesting sites. The summer low flow proposed in the 2002 biological opinion has no apparent advantage in the successful reproduction of the birds as compared to present management practiced by the Corps. The natural hydrograph was very damaging to the least terns and piping plovers and accordingly terns and plovers were not typically resident to the sandbars but were migrant visitors.
Since the construction of the main-stem dams, the hydrograph of the Missouri River has been significantly changed. The largest change is that large flood events associated with the June rise, in general, have been moderated. In reference to the sandbars in the Gavins Point to Ponca, Nebraska, reach, the greatly reduced flooding of sandbar islands has resulted in the birds now nesting and reproducing commonly. During the last two decades, the USACE has implemented a nearly ‘flat’ hydrograph by strictly controlling the discharges from Gavins Point Dam. This management scheme has resulted in increased fledging rates and correspondingly an increase in the population of the birds. The present hydrograph is very beneficial to the least tern and piping plover. The results of this investigation indicate that the operation of the dams doesn’t cause jeopardy to the least tern and piping plover.