C150063 Feasibility Study

• Contract/PO #: C150063
• Contractor Name: Jackson Lake Reservoir & Irrigation Company
• Contract Type: Loan
• Water District: 1
• County: Morgan
• Bill Number: SB 94-29
• Loan Projects - Doc Type: Feasibility Study

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RESERVOIR YIELD

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The firm or safe yield of a reservoir is generally defined as "the maximum
quantity of water that can be guaranteed during a critical dry period." The safe yield of
Jackson Lake Reservoir was not evaluated by analysis of the available flow of water in
the South Platte River due to the extensive size and complexity of the basin and water
use. The Jackson Lake Reservoir yield was evaluated by review of the past record of
filling the reservoir maintained by the Colorado Division of Water Resources7 to
ascertain whether the reservoir is filled to capacity in wet and dry years. A summary of
the maximum and minimum reservoir water levels is shown on Figure 2 for the years
1949 through 1999. Note that the records are not complete for some of the years due
to incomplete records in the State Engineer's Office. It is noted that Jackson Lake
Reservoir has the most senior large reservoir storage decree on the main stem of the
South Platte River and, as.such, it is generally expected to fill each year.

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From the records shown on Figure 2 it can be seen that the reservoir is filled to
near capacity during most years and undergoes significant drawdown each year. In the
51 years of record presented, the reservoir has filled every year essentially to the
maximum storage level at gage height 30.0 feet. The lowest maximum fill level was to
gage height 26.6, which occurred in 1949, a year that was a high-flow year. Because
the State Engineer's records are only for end-of-month readings, it is probable that
under the several years where maximum storage was not noted, the maximum reservoir
level occurred during the middle of the month.

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The reservoir is normally drawn down during the irrigation season as shown on
Figure 2. The reservoir is empty at about gage height 13.0. The Reservoir Stage-
Storage Capacity chart is shown on Table 4. The 1948 capacity data is used for daily
operations of the reservoir and includes adjustments for seepage and evaporation losses
during the irrigation season, and therefore does not reflect the actual water volume in
storage. The Filing Map capacity data is the best available information on the reservoir
storage capacity.

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