WSP09032

• File Number: 8407
• Description: Platte River Basin - River Basin General Publications
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 2/27/1991
• Author: BOR
• Title: Annual Operating Plans - North Platte Project Water Years 1990-1991
• Water Supply Pro - Doc Type: Report/Study

Duri ng April, the snowpack increased to 92 percent of average and the
April-July forecast was 205,000 acre-feet. April precipitation was above
normal at 120 percent of average and April inflows were 93 percent of
average.

Reservoir storage reached the year's low of 56,316 acre-feet on April 18.
A target storage of 65,000 acre-feet at the end of April is desired for the
most probable runoff condition and 70,000 acre-feet for the minimum runoff
condition, Due to the high Dillon depletions, below normal runoff, and
projected natural flow demands, a minimum bypass release of 60 ft3/s was
initiated on May 9, 1990. May was cool and precipitation was 86 percent
of average. Computed reservoi r i nfl ow increased from 250 ft3/s to near
950 ft3/s by the end of the month.

Dillon Reservoir filled to capacity on June 10 and started to spillover
the uncontrolled spillway. Spill at Dillon reached a maximum of 1,149 ft3/s
by June 14. The spill ended on July 15 for a total of 29,933 acre-feet
spilled in water year 1990. Dillon Reservoir ended the water year at a
storage level of 253,678 acre-feet.

Computed inflow to Green Mountain Reservoir peaked for the year at
2,051 ft3/s on June 12. The minimum bypass release of 60 ft3/s continued
through June 14. Turbine releases averaged only 124 ft3/s from June 15
through June 30 in order to increase reservoir storage near to a physical
fill (154,645 acre-feet), June was hot and dry, as precipitation was only
35 percent of average.

During July, turbine releases were regulated to attain the target storage
level between 152,000 acre-feet and 154,645 acre-feet, which fills the
replacement pool (52,000 acre-feet) and the power pool
(100,000 acre-feet). Turbine releases averaged 296 ft3/s during July.
Green Mountain Reservoir inflow decreased to near 350 ft3/s by mid-July and
to near 250 ft3/s by the end of the month. Maximum reservoir storage was
152,835 acre-feet on July 17. Actual April-July runoff was
218,300 acre-feet, 82 percent of average, and 106 percent of the May 1
forecast,

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Release for natural flow shortage was called for on the Colorado River on
July 27 at the Shoshone Powerplant when the flow dropped below 1,408 ft3/s
and then on July 30 when the flow at the Cameo gage dropped below
2,260 ft3/s. Turbine releases were increased to approximately 450 ft3/s by
the end of the month to meet the Cameo call. Due to the very dry
conditions along the Colorado River, releases for natural-flow shortage,
bypass of inflow, and CBT Project replacement continued from Green Mountain
Reservoir for the remainder of the water year, and were as high as
1,317 ft3/s on September 1. The recorded flow at Dotsero for the Colorado
River was 950,130 acre-feet for water year 1990, which was only 62 percent
of average and the fourth lowest on record since 1900 (exhibit 3). The
1987-1990 recorded flow at the Dotsero gage on the Colorado River totaled
4,463,900 acre-feet and this was the lowest consecutive 4 year total since
flow records started in 1900,

The August and September inflows were 70 percent of average and 117 percent
of ~verage, respectively. September was wet at 119 percent of average

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