WSP00928

• File Number: 8220.105.I
• Description: Colorado River-Water Projects-Navajo-Environmental Studies
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 10/1/2001
• Author: BOR
• Title: Navajo Dam EIS-Biological Assessment-Preliminary Draft
• Water Supply Pro - Doc Type: EIS

provide suitable conditions for the endangered species. They can be achieved by using the
operating criteria for Navajo Darn outlined ... By recommending operating rules, natural
variability in the hydrograph is maintained and decision making for annual releases from Navajo
Darn is simplified." (Holden, 1999).

Flow recommendations are targeted for the 180 miles of the San Juan River between the Animas
River confluence and Lake Powell.

By applying the operating rules in the flow recommendation report, the depletions shown in the
table in Attachment B (for the proposed plan) can be made and the flow recommendations met.
These depletions include the Animas LaPlata Project and completion of the Navajo Indian
Irrigation Project.

A summary of flow recommendation conditions follow:

A. Category:
Duration:
Frequency:
B. Category:
Duration:
Frequency:
C. Category:
Duration:
Frequency:
D. Category:
Duration:
Frequency:
E. Category:
Timing:
F. Category:
Level:
G. Category:
Control:

Flows> I 0,000 cfs during runoff
A minimum of 5 days between March I and July 31
20 % of years, with maximum number of years between events 10 yes.
Flow>8,000 cfs during runoff
A minimum of 10 days between March I and July 31
33% of years, with maximum number of years between events 6 yes.
Flows>5,000 cfs during runoff
A minimum of21 days between March I and July 31
50% of years, with maximum number of years between events 4 yes.
Flows> 2,500 cfs during runoff
A minimum of 10 days between March I and July 31
80% of years, with maximum number of years between events 2 yes.
Timing of peak flows
Within 5 days ofhistorical mean peak date of May 31
Target base flows (mean weekly)
500 cfs target from Farmington to Lake Powell with 250 cfs minimum
release from Navajo Darn. Range-500-1,000 cfs.
Flood control releases
Handle as a high magnitude, short duration spike and release when flood
control rules require, except that the release shall not occur earlier than
September I. If an earlier release is required, extend duration.

Additional details on the recommendations are included in the Flow Recommendation Report.

HYDROLOGY AND WATER QUALITY

This section of the biological assessment addresses changes to hydrology that would occur with
implementation of the proposed plan for reservoir operations. In general, the changes will

9