9344

• UCREFRP Catalog Number: 9344
• Author: Camp Dresser & McKee Inc.
• Title: Replacement of the Plateau Creek Pipeline.
• USFW Year: 1996.
• USFW - Doc Type: Denver.
• Copyright Material: NO

To illustrate the effects of the Section 7 reductions on the Colorado River baseline in an avera e
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year, the average year monthly reductions were subtracted from the monthly Historic Colorado River
baseline flows (discussed in Section 3.3.2) at the top of the 15-mile reach. Figure 3.6 highlights the
montlily effects of these reductions with the most substantial effects clearly indicated in May, June,
and July.
3.3.3.3 Adjustment for Annual Hydrologic Variation
To account for yearly hydrologic variations over the period studied (1975 through 1993), the
monthly Section 7 flow reductions for the average hydrologic year were adjusted up or down (for
a wet or dry year) by the ratio of the historic yearly annual Colorado River flow to the historic
average annual Colorado River flow for all years in the study period. The same factor was applied
to the reduction in each month of each year under the assumption that monthly variations were
already reflected in the project distributions. The USFWS concurred with this approach during
informal Section 7 consultation meetings. The approach resulted in specific Section 7 flow
reductions to apply to the historic Colorado River Flow at the top of the 15-mile reach in each month
of each year. The resulting subtraction became the adjusted historic Colorado River baseline flow
for each of the 19 years.
For the Windy Gap project Section 7 depletions, available historic portions in May, June, and July
were obtained from the Bureau of Reclamation for the years 1985 through 1993. These historic
depletions are already reflected in the unadjusted historic Colorado River baseline. Subtraction of
the entire depletion again would have resulted in a double counting of historic depletions, so it was
necessary to remove the historic portion from the total Section 7 monthly. flow reductions in the
years for which records were available. The adjustment process is documented in Attachment B for
detail. Table 3.9 summarizes by year the final Section 7 flow reductions (including Windy Gap
adjustments) which were applied to the historic Colorado River flows at the top of the 15-mile reach
to obtain the adjusted Colorado River baseline.
3.3.4 Project Flow Reductions In The 1 S-Mile Reach
To evaluate ro'ect related effects on Colorado River tr amflows at the to of the 15-mile reach
p ~ s e p ,
withdrawals from the basin by the District which result in flow reductions were computed under both
the baseline and future demand scenarios. The baseline demand for the simulation was set at 6945
ac-ft/yr, which is the average volume of finished water the District used during the period from
1984-1987 (as discussed in Section 3.2.2). Future demands of the District's customers in the year
2045 are estimated to be approximately 28,589 ac-ft/yr based on the Scenario C demand projections
(see Section 2.0). Since the goal in this task was to evaluate the reduction in flows under a wide
-range of hydrologic conditions (i.e., the 1975-1993 analysis period), the District's raw water system
model was used to simulate the operation of their system under both baseline and future demands.
The computation of flow reductions is provided in Attachment C. Reductions were calculated as
monthly withdrawals by the District over the 19-year simulation period first under baseline demands
(pages 1-10 of Attachment C), under future demands (pages 11-20), and finally as net withdrawals
(pages 21-30), which are simply the baseline results subtracted from the future results. Flow
CDM Camp Dresser & McKee
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