WSP04963

• File Number: 8220.105.I
• Description: Colorado River-Water Projects-Navajo-Environmental Studies
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 4/1/2002
• Author: BOR
• Title: Navajo Dam EIS-Summer Low Flow Test Report-San Juan River New Mexico and Utah
• Water Supply Pro - Doc Type: Report/Study

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TROUT POPULATION AND AOUA TIC HABITAT

Outcome

The Test had minimal short-term effects on the aquatic
ecosystem of the San Juan River. Decreases in river stage,
wetted perimeter,3 and average habitat depth did not result
in direct mortality of fish. However, habitat of fish was
clearly reduced, in all likelihood forcing fish to use the
deeper runs and pools for resting and escape cover; this
could result in crowding and possible stress of fish over an
extended period of time. Reductions in wetted perimeter
would decrease aquatic insect and other fish food pro-
duction, but an adequate food supply would remain because
of existing insect density and the moderate degree of
reduction in wetted area,

Over a longer period, the reduction of habitat and an
associated decrease in water quality could have adverse
effects on the trout fishery. According to the New Mexico
State Engineer's Office, the Bloomfield Irrigation District
is entitled to divert into the Citizens Ditch approximately
160 cfs; if this occurred, impacts would increase down-
stream, During the Test, habitat was clearly reduced in this
area, possibly resulting in overcrowding in addition to
causing stress to the fish over an extended period,
Reduction in depth of riffles could limit available secure
feeding areas for such drift feeders as trout and such bottom
scrapers as native suckers.

In general, prolonged low flows or frequent low-flow
periods could reduce fish populations as a result of

l Length of wened channel is the horizontal distance of the river between two
points perpendicular to flow.

Summer Low Flow Test Report - April 2002

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CDS9?