Platte River Management Joint Study Final Report

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, 25 the river stage precluded emergence of suitable sandbars for nesting. By late June, river stage was lowered causing sandbars to be exposed and least terns initiated nesting. Heavy rains fell on 30 June and in concert with irrigation flows placed back in the channel caused the river stage to rise dramatically (Table 10). Flows during late June and early July at Overton, Nebraska, ranged from 111 cfs on 27 June to 3,881 cfs on 1 July. Flows at Odessa, Nebraska, ranged Irom 11 cfs. on 29 June to 2,600 cfs on 2 July. Flows at Grand Island ranged from 99 cfs on 28 June to 2,510 cfs on 4 July (USGS, unpubl. data). All known nests in the Big Bend reach of the river channel were inundated from.rising water by 4 July. - . ' Downing (1980) similarly reported that the Platte River breeding population was affected by high flows during a survey conducted in late June 1975. The minimum flow for any day during the summer prior to Downing's survey was 280 cfs at Overton, 388 cfs at Grand Island and 482 cfs at Duncan (USFWS 1987a). U.S Geological Survey records indicate a flow of 323 cfs at Grand Island in 1984 atthe time human disturbance of a colony near Alda occurred. Recreational disturbance of a nesting colony near Alda on 6 or 7 July and 20 or 21 July 1985 resulted in direct- mortality of young. Grand Island gage records of flows on those dates were 633, 564, 636 and 715 cfs respectively (USFWS 1987a). All-terrain vehicle use of the Platte River channel at Grand Island was also observed by USFWS and Bureau personnel on 25 July 1986; U.S. Geological Survey data indicate a flow of.601 cfs at tne Grand Island gauge on that day. . Downing's (1980) observations appear to be consistent with USGS gaging records which reflect a sudden rise in flows at that time (950 cfs on 16 June 1975; 5300 cfs on 23 June 1975).. Low flows are known to allow increased access to the river channel by reGreationists (NGPC unpubl. data). Low flows and high water temperatures can result in the death of forage fish consumed by least terns as occurred in June 1988 (USFWS, unpubl. data). Human Disturbance - Human activities near least tern and piping plover nesting areas can pose a regular threat to breeding success (Sidle et. a1..1988). Human disturbance may prevent use of an otherwise suitable site, or cause nest abandonment or direct mortality of eggs and.young. NGPC (1982) reported that open riverine sandbars are used for summer recreational activities such as picnicking, sunbathing, fireworks displays., driftwood collection and other activities. Downing (1980) observed vehicle tracks on most emerged sandbars in the Platte River in 1975. Ducey (1981) similarly reported human activities near most riverine colonies surveyed, including swimming, campfires, wading, hiking on sandbars, -as well as vehicular use. Heavy vehicular use at one site apparently caused adult least terns to abandon their nests.