WSP00432

• File Number: 8441.700
• Description: Colorado Big Thompson Project - Annual Operating Plans
• Basin: South Platte
• Water Division: 1
• Date: 1/1/1990
• Title: Annual Operating Plans - Colorado Big Thompson Project - Water Year 1990-1991
• Water Supply Pro - Doc Type: Report/Study

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Duri ng Ap~il, 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 istorage 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 proba,ble runoff condition and 70,000 acre-feet for the minimum runoff
condition.: Due to the high Dillon depletions, below normal runoff, and
projected inatural flow demands, a minimum bypass release of 60 ft3/s was
initiated ion May 9, 1990. May was cool and precipitation was 86 percent
of averag~. Computed reservoi r i nfl ow increased from 250 ft3/S to near
950 ft3/s ~y the end of the month.

Dillon Res:ervoir filled to capacity on June 10 and started to spill over
the uncontrolled spillway. Spill at Dillon reached a maximum of 1,149 ft3/s
by June 141. 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 lerel of 253,678 acre-feet.

Computed inflow to Green Mountain Reservoir peaked for the year at
2,051 ft3/~ on June 12. The minimum bypass release of 60 ft3/s continued
through Juhe 14. Turbine releases averaged only 124 ft3/S from June 15
through Ju~e 30 in order to increase reservoir storage near to a phYSical
fill (154,~45 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 betw~en 152,000 acre-feet and 154,645 'acre-feet, which fills the
replacement pool (52,000 acre-feet) and the power pool
(100,000 aqre-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 25q ft3/s by the end of the month. Maximum reservoir storage was
152,835 a~re-feet on July 17. Actual April-July runoff was
218,300 acrie-feet, 82 percent of average, and 106 percent of the May 1
forecast. '

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 o^n July 30 when the flow at the Cameo gage dropped below
2,260 ft3/S1 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 ~long the Colorado River, releases for natural-flow shortage,
bypass of inflow, and CBT Project replacement continued from Green Mountain
Reservoir f,or the remainder of the water year, and were as high as
1,317 ft3/s ion September 1. The recorded flow at Dotsero for the Colorado
River was 9~0,130 acre-feet for water year 1990, which was only 62 percent
of average ~nd 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 a~re-feet and this was the lowest consecutive 4 year total since
flow recordS started in 1900.

The August ~nd 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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