Abstract
The U.S. Geological Survey (USGS), in cooperation with the Shoshone-Bannock Tribes, surveyed
the bathymetry and selected above-water sections of Blackfoot Reservoir, Caribou County, Idaho, in
2011. Reservoir operators manage releases from Government Dam on Blackfoot Reservoir based on
a stage-capacity relation developed about the time of dam construction in the early 1900s. Reservoir
operation directly affects the amount of water that is available for irrigation of agricultural land
on the Fort Hall Indian Reservation and surrounding areas. The USGS surveyed the below-water
sections of the reservoir using a multibeam echosounder and real-time kinematic global positioning
system (RTK-GPS) equipment at full reservoir pool in June 2011, covering elevations from 6,090 to
6,119 feet (ft) above the North American Vertical Datum of 1988 (NAVD 88). The USGS used data
from a light detection and ranging (LiDAR) survey performed in 2000 to map reservoir bathymetry
from 6,116 to 6,124 ft NAVD 88, which were mostly in depths too shallow to measure with the
multibeam echosounder, and most of the above-water section of the reservoir (above 6,124 ft
NAVD 88). Selected points and bank erosional features were surveyed by the USGS using RTK-GPS
and a total station at low reservoir pool in September 2011 to supplement and verify the LiDAR
data. The stage-capacity relation was revised and presented in a tabular format. The datasets show
a 2.0-percent decrease in capacity from the original survey, due to sedimentation or differences in
accuracy between surveys. A 1.3-percent error also was detected in the previously used capacity table
and measured water-level elevation because of questionable reference elevation at monitoring stations
near Government Dam. Reservoir capacity in 2011 at design maximum pool of 6,124 ft above
NAVD 88 was 333,500 acre-ft.
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First posted September 5, 2012
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