WSP05236

• File Number: 8220.106
• Description: Animas-La Plata
• State: CO
• Basin: San Juan/Dolores
• Water Division: 7
• Date: 2/26/1996
• Title: Final Biological Opinion - 1996
• Water Supply Pro - Doc Type: Biological Opinion

Discussion

Stage A, including the key water management decisions discussed earlier, was
analyzed using the same hydrologic model that was used for the analysis of the
full Animas-La Plata Project. The period of record selected for the analysis
was 1935 to 1962. From this period, two wet, two dry, and two average years
were selected for analysis based upon water volumes during the spring runoff.
Wet years selected were 1938 and 1949, dry years were 1946 and 1951, average
years were 1936 and 1945. Figures 1-18 at the end of this appendix showing
the flow patterns at three locations resulting from the implementation of the
reasonable and prudent alternative.

1. Reoperated at Current (baseline) Level of Depletion

The first step in the analysis is to investigate the opportunities for
improving the flow pattern by reoperating Navajo Dam. The level of
depletions is identical to the Section 7 baseline (listed in Table 2) in
the Biological Opinion. Under this scenario, the minimum release below
Navajol)am for trout was reduced to 300 cfs and a 300,000 AF release for
endangered fish was made in May and June. Under this scenario, the
release for endangered fish was able to be made in 96 percent of the
years.

a. Magnitude of Peaks Compared to Historic

The level of depletion in the San Juan River Basin has increased
since the pre-Navajo Dam era and Navajo Dam is storing water during
the peak runoff; therefore, the peaks are lower than historic.
During wet years, for instance, the historic peaks in 1938 and 1949
were 11,478 cfs and 12,566 cfs respectively. Under the current level
of depletion, Navajo Dam can be operated to achieve peaks of 9,317
cfs and 10,460 cfs respectively. These peaks are comparable to the
1987 peak of 9,026 cfs (Figure 20).

During average years, the historic peaks in 1936 and 1945 were 6,905
cfs and 7,419 cfs respectively. Under the current level of
depletion, Navajo Dam can be operated to achieve peaks of 6,905 cfs
and 7,419 cfs respectively. While comparable to the historic spring
peaks, these flows are significantly less than the 1987 spring peak.

In dry years, the spring peak can be enhanced significantly with
storage releases from Navajo Dam. Historic peaks in 1946 and 1951
were 3,427 cfs and 3,169 cfs respectively. Under the current level
of depletion, Navajo Dam can be operated to achieve peaks of
4,552 cfs and 4,217 cfs respectively. These peaks are higher than
historic peaks for dry years and may represent an improvement to
habitats in dry years or could be spread out over a longer period.

b. Shape of Hydrograph (ascending/descending limbs)

During wet years, Navajo Dam is refilling, and the ascending limb of
the hydrograph falls short of historic flows (see figures). Since

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