Navajo Reservoir Operations Vol. II

• File Number: 8220.105.I
• Description: Colorado River-Water Projects-Navajo-Environmental Studies
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 9/1/2002
• Author: BOR
• Title: Navajo Dam EIS-Draft EIS-Navajo Reservoir Operations-Volume II
• Water Supply Pro - Doc Type: EIS

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Flows> ] 0,000 cfs for 5 days or more need to occur in 20 % of the years
on average for the period of record 1929-1993. Maximum number of
consecutive years without meeting at least a flow of 9,700 cfs (97% of 10,000
cfs) within the 65-year period of record is 10 years.

Flows above 10,000 cfs provide significant out-of-bank flow, generate new
cobble sources, change channel configuration providing for channel diversity,
and provide nutrient loading to the system, thus improving habitat
productivity. Such flows provide material to develop spawning habitat and
maintain channel diversity and habitat complexity necessary for all life stages
of the endangered fishes. The frequency and duration are based on mimicry
of the natural hydrograph, which is important for Colorado pikeminnow
reproductive success and maintenance of channel complexity, as evidenced
by the increase in the number of islands following high flow conditions.
Channel complexity is important to both Colorado pikeminnow and
razorback sucker.

Flow> 8.000 cfs during runoff period.

A minimum of]O days between March] and July 31.

Flows> 8,000 cfs for 10 days or more need to occur in 33% of the years
on average for the period of record ]929-1993. Maximum number of
consecutive years without meeting at least a flow of7,760 cfs (97% of 8,000
cfs) within the 65-year period of record is 6 years.

Bankfull discharge is generally between 7,000 and 10,500 cfs in the San Juan
River below Fannington, New Mexico, with 8,000 cfs being representative
of the bulk of the river. Bankfull discharge approximately I year in 3 on
average is necessary to maintain channel cross-section. Flows at this level
provide sufficient stream energy to move cobble and build cobble bars
necessary for spawning Colorado pikeminnow. Duration of 8 days at this
frequency is adequate for channel and spawning bar maintenance. However,
research shows lil positive response of bluehead sucker and speckled dace
abundance with increasing duration of flows above 8,000 cfs from 0 to 19
days. Therefore. the minimum duration was increased from 8 to 10 days to
account for this measured response. Flows above 8,000 cfs may be important
for providing habitat for larval razorback sucker if flooded vegetation and
other habitats formed during peak and receding flows are used by the species.
This flow level also maintains mimicry of the natural hydrograph during
higher flow years, an important feature for Colorado pikeminnow
reproducti ve success.

SJRIP Biology Committee
May 1999

8-6

Executive Summary
Flow Report

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