WSP00190

• File Number: 8407
• Description: Platte River Basin - River Basin General Publications
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 6/2/1999
• Author: URS Greiner Woodward
• Title: Documentation of Existing Conditions in the Central Platte Valley - Delivery Order Number 86
• Water Supply Pro - Doc Type: Report/Study

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Platte River. Although nest sites also had higher mean and maximum elevation than systematic
sample sites, these differences were not statistically significant in either river reach. On the
central Platte River, the mean elevation of nests was less than the mean site elevation at half of
the nest sites (FWS 1997).

These differences between nest and systematic sample sites and between the lower and central
Platte River suggest that habitat availability is limited in the river channel along the central Platte
River. Along the central Platte River, a greater number of least terns nest on sand and gravel
spoil piles than on the river (Table 4-3). Many researchers believe high usage rates of the sand
pit habitat by least terns is an indication that adjacent riverine habitat is limited.

Low flows followed by sudden peaks in early and late July resulted in the inundation of eight of
the thirteen nests surveyed at central Platte River sites in 1988. The median elevation of these
nests equates to a stage of about 2,895 cfs at the Grand Island gage. Flows higher than 2,895 cfs
during June 16 to August 31, the latter half of the nesting season, occurred in 18 of the 30 years
from 1959 to 1988, suggesting that nest inundation is likely to be a common occurrence given
present habitat conditions (FWS 1997).

These facts suggest that those channels of the central Platte River wide enough to be attractive to
least terns and piping plovers are too flat to provide safe nesting substrate. Higher portions of the
riverbed that might have provided safe nesting substrate in the past have become vegetated
islands.

Nesting at Sand and Gravel Mining Operations

Sidle and Kirsch (1993) analyzed the use of sand pits by least terns along the entire Platte River.
There are about 255 sand and gravel mining sites along the Platte, Loup, and North Loup rivers;
the majority occurring along the central Platte River. Most (99 percent) were within I mile of
the rivers, and 178 provided no suitable nesting substrate. Few sand and gravel mining
operations occurred along the Loup and North Loup rivers. The number of mining sites with
suitable nesting habitat was similar along the central and lower Platte River, but most unsuitable
sites occurred along the central Platte River.

However, within the Central Platte study area, most least terns (61 to 94 percent) nested at sand
and gravel pits, 1988 to 1998 (Table 4-3). Alternatively, during the same period only 14 to 40
percent of least terns nested at mining sites along the lower Platte River. In any year, more
suitable mining sites were used along the central than the lower Platte River. As discussed
previously, the high usage rate of sand pit habitat may be an indication that adjacent riverine
habitat is limited. Numbers of least terns were less correlated with size of the sand pits on the
central than the lower reach of the Platte River.

Size and shape of suitable sand and gravel pit habitat varied. Some had roughly linear lakes with
chains of sand and gravel piles jutting from the bank. Others contained circular lakes with sand
and gravel on one or more banks with an occasional peninsula of sand and gravel extending into
the lake. Three sand pits contained an island of sand and gravel in lakes. Sites along the Platte
River ranged in size from 1.5 to 197 acres in size and averaged 59 acres (n = 57). The sand and

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