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<br />128
<br />
<br />CALIFORNIA FISH AND GAME
<br />
<br />Effects of Ohanges in Silt and Flow
<br />
<br />It is impossible to consider the cha~ges in flow and silt-load ~n the
<br />Colorado River separately, and these brmg about other chang~s m t~e
<br />physical character of the strea~. In genera~, the new.dams with their
<br />reduction of flooding and silting affect gr~Ient, -yeloClty, .character of
<br />the bottom material, and marginal vegetatIon. W~th the dI~appearance
<br />of floods the flow can be carried on a reduced gradient and m.a redu~ed
<br />cross-sectional area, The bed becomes flattened ~nd mfirgmal areas
<br />become overgrown with vegetation. As the reser:roIrs fill .m, the stream
<br />~_.~~:::-'._ above becomes aggraded. Bel()W'.<la.m~ retrogr:esSlOn sets .m. A further
<br />change is the gradual increase of coarse material on the'l'lver bottom,
<br />
<br />" , A th b ws it willI becom'
<br />FIG, 58, The formation of a "Jake" in I?miabtur:'f s thee m~fn~hannel l'ppH pan
<br />invested with vegetation and cut of1 the ay rom '
<br />
<br />of Lake Havasu, Feb. 1942.
<br />
<br />At present the section immediately below Boulder Dam is I1l1d:~
<br />O'oing retrogression and its bed material has become coarse by rrm~'l_
<br />~f the fines. (See Stevens, 1938, Fig. 4,) "Fro~ Boulder Dam :,'\ t~e
<br />low Beach the Colorado River is no\V paved .mth c?,arse matena R n,
<br />smallest sizes of which run about two or three mches. (Letter of"s'tht'
<br />MacDonald, to the author, March 2,1944.) Downstream from Da\1
<br />tendency will be for the stream bed to become coarser also., p ker
<br />AO'O'radation can be expected above Parker Dam. Between ar '_1
<br />ee . 1 t . Imper1i1'
<br />-and Imperial Dam there is an area of potent.la re rogressIon. the
<br />Dam itself will quickly silt up. FrOID ImperIal Dam to Laguna 1?aDl 8V
<br />i".ection may be built up by desilting operations and channel flushmg' Dl ~
<br />~ossiblv be resorted to. The fate of the river below Laguna seem~ to
<br />uncertain, but ultimately the delta IDay extend further upstream,
<br />
<br />THE FISHERY OF THE LOWER COLORADO RIVER
<br />
<br />129'
<br />
<br />---....:...".
<br />
<br />,"crd()f explanat~on as .to the" lak~' , along the Colo:adi) isperti-~'~~i~-=s:~
<br />~.Where the rlver Wldensout, eIthe:r Ilaturally as Ins-valley or.~~:""?__.
<br />" ~above a dam, quieter water areas exist to the side of the ma:m-'-~,.c:;'--'
<br />W;'As the stream drops its load it spreads out and sand (silt) bars:'~
<br />", ,which partially cut off the expanded areas near the borderS ___:~~_.,
<br />.".in channel. Growth of these bars, especially when accompanied
<br />establishment of vegetation on them, may eventually result in a
<br />lake or slough which may become quite distinct from the main
<br />;: the lake is shallow and silt continues to be deposited in it, it may
<br />eM into a marsh by the growth of bulrushes and cattails. How-
<br />.:the expansions are rather deep and the bars extend'so completely "
<br />t;::..eut them~;' sizableareag-:bearingsom~:rtacustrin.e::charac=:--=::=--=-.
<br />-;~e created, with quiet waters much clearer than the channel.
<br />~:a.t Martinez Lake, they are filled with drowned vegetation. All
<br />-' long the river are rather impermanent and may be destroyed
<br />in ~ow, shifts in the river's course, or silt deposition. At
<br />~ervoir at least one large lake has already disappeared, and
<br />@ly lake will exist is unknown.
<br />,'(1938) has discussed the effects of the dams on the lower
<br />liis conclusion may be repeated here, ,'. · · it is well to
<br />,at,it will take many years to establish the new regimen, pos-
<br />iter of 30 to 50 years, that no one can predict with accuracy
<br />, pen." One must realize that the Colorado is still in a state
<br />tgenerally speaJcing the changes appear beneficial to the fish~ ""., ----,
<br />hole. ~~ Wherever there is clearer water, more uniform flow
<br />d more permanent sediments, slackened velocity and marginai
<br />-the establishment of aquatic life is more certain.
<br />
<br />Air Temperature
<br />
<br />"~ptionally high air temperatures prevail in the basin of the lower
<br />tJi Yuma, for example, has a mean annual temperature of about
<br />:~d a ma.ximum temperature of 1200 F, has been recorded llere,
<br />,.oon temperatures reach 1000, on an average, from June 8th to
<br />,ber 13th and 1050 from June 26th to August 16th" accordin~
<br />, ,on (1942), The high air temperatures permit an all year round
<br />tU~al season in the fertile valleys along the river, Thus there is
<br />j;;irrigation 365 days of the year. This has a direct effect on the
<br />I.the region as the main canals flow throughout the year and thus
<br />;)'a:ther permanent habitat for fish. The direct effect of air
<br />ure on the duration of the fishing season can be easily perceived.
<br />~9.)
<br />ally the trend of air temperatures is directly reflected in the
<br />ateI' temperatures. (See Fig. 59.) The river becomes very
<br />". g the summer months, and never drops to a very lo~
<br />e.
<br />
<br />.' Water Temperature
<br />
<br />.J:LlY'long term records of water temperature for the lower river
<br />the author are presented in this report. Any of the graphs
<br />,,~g, 61) can be used to point out the general range and trend
<br />
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