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<br />Post dam sediment load of the
<br />Colorado River was reduced by about
<br />90% from retention in Lake Powell,
<br />and removal of sediments through
<br />Grand Canyon has been substantial as
<br />a result of scouring and clear water
<br />releases, This has greatly reduced
<br />allochthonous material in the
<br />Colorado River in Grand Canyon and
<br />modified availability of aquatic food
<br />supplies, while converting the system
<br />from heterotrophic to autotrophic
<br />energy sources. Clear, cold releases
<br />probably eliminated many of the invertebrates
<br />native to the river, leaving only nearctic species,
<br />The degree and frequency of turbidity has been
<br />reduced and sediment replacement is now primarily
<br />from the Paria River and the Little Colorado River
<br />(LCR), Daily fluctuations maintain turbidity in the
<br />mainstem and a regular supply of food in the
<br />system. Water clarity is substantially increased
<br />when flows are low and constant (Fig. 3).
<br />
<br />2 . Executive Summary
<br />
<br />become fully evident until about 1970
<br />(Fig. 2), when Lake Powell reached
<br />sufficient depth to stratifY. Maximum
<br />longitudinal warming of the Colorado
<br />River below the dam now occurs in
<br />late summer at a rate of about IOCI51
<br />kIn.
<br />
<br />DISTRIBUTION
<br />
<br />The present distribution of humpback
<br />chub in Grand Canyon is related to
<br />mainstem temperature, locations of
<br />warm tributaries and springs,
<br />occurrence of suitable adult habitat,
<br />and food availability. Humpback
<br />chub were found in 308 kIn of the
<br />mainstem Colorado River, from
<br />,Shinumo Wash (RM 29) to Granite
<br />Springs Canyon (RM 220), as nine
<br />persistent aggregations in which 94%
<br />of 6,294 chubs (92% ofYOY, 94% of
<br />juveniles and 98% of adults) were
<br />captured (Fig, 4), Four of these
<br />aggregations were associated with
<br />warm tributaries (LCR, Bright Angel
<br />Creek, Shinumo Creek, Havasu
<br />Creek), and two with warm springs
<br />(Fence Fault Springs, Pumpkin
<br />
<br />Final Report
<br />
<br />3D
<br />
<br />2S
<br />
<br />E>>
<br />!
<br />:l
<br />! 16
<br />!
<br />E
<br />GI 10
<br />I-
<br />
<br />
<br />IIl66
<br />
<br />1_
<br />
<br />1870
<br />
<br />1875
<br />
<br />,.
<br />
<br />CaJendar Year
<br />
<br />Fig. 2. Mean monthly water temperature at Lees Ferry following
<br />closure of Glen Canyon Dam and impoundment of Lake Powell on
<br />March 13, 1963. Monthly means are based on measurements at 15-
<br />min intervals. Suitable spawning temperature range for humpback
<br />chub is shown at 16-220C.
<br />
<br />Spring), indicating that fish were attracted to
<br />thermal areas in the cold river. Four aggregations,
<br />including the two largest, were associated with
<br />unique geomorphic reaches (LCR, Lava to Hance,
<br />Stephan Aisle, Middle Granite Gorge),
<br />characterized by numerous debris fans and
<br />associated recirculating eddies.
<br />
<br />Longitudinal warming in summer of IOCI51 kIn
<br />increased mainstem temperature from an average of
<br />
<br />30,000
<br />
<br />6
<br />
<br />_Flow
<br />;\,,--.-- Secchi Depth I- 5
<br />
<br />I \
<br />
<br />
<br />,
<br />
<br />
<br />
<br />.
<br />
<br />
<br />. i
<br />.'i~_
<br />
<br />May 10-23, 1991
<br />
<br />25,000
<br />
<br />20,000
<br />
<br />4fe
<br />n
<br />n
<br />::
<br />30
<br />ID
<br />'0
<br />-
<br />~
<br />
<br />~
<br />.2.
<br />~ 15,000
<br />o
<br />u:
<br />
<br />" ...
<br />..
<br />
<br />\
<br />
<br />'?-
<br />
<br />2~
<br />
<br />10,000
<br />
<br />'"
<br />
<br />5,000
<br />
<br />... -"...
<br />"0-" ..
<br />. .J 0 . h'_... .- .
<br />, -.
<br />.----.
<br />
<br />,
<br />o 10 11 12 13 14 15'16 -'17-'18 19 20 21 '22 23
<br />Date
<br />
<br />o
<br />
<br />Fig. 3. Relationship of flow to Secchi depth during a transition of high
<br />fluctuating releases (7,000-25,000 cfs) to constant 5,000 cfs, May 10-
<br />23,1991.
<br />
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