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,. ;.. --~.:_ „®,~~ CHAAf GE 4A9 . F#iant; 19©3), ale-and . ett .~.~: 197Q7I 3L', x' 13~ 4E? ing~ In'the processof 3ierr2 Nevada foothill ng, shallow, exposed the straightening and .. of flood control by ions of Rush Creek,.. ain fewer fish overall,- scwlpinand speckled.- da~~man~ to-maintain large populations in the channelized 'seCtlans (Mcfyler in.: press.; Tl-e .decrease in size and numbers of fish were caused by the reduction of habitat diversity, especially the elimination.of pools...Such effects ace typical ofothez streams as well: Tltecdredged_channels of the Sacramento•San JoaquinDelta_are an example:.of sty=channel. alterations on a mammoth scale. While fish are often .surprisingly nnnresous in the channels,-habitat variety, and thus the number: of species;. is limited.-. Id;;partirulaz, the. elimination of mast. of the tule marshes through which the old _ channels. meandered is probably the main reason for the extinction of the. thicktail_. clst~h~nd-.for other. native fishes having become only minor components. of the fish: fanna~ Sianilaz effects. were .observed .when sloughs along the lower Colorado .River-: we~rrirained as part of a lazge channelization project (Beland,1953a~. TTe: mast dramatic type of channel. alteration in recent decades. has been -the _ construction of dams and reservoirs, an activity that. bloomed with the advent of the Central~ Valley Project. in the 1930s and has only slightly slowed -dawn at the. present.: time.: Qne..of .the. most severe effects of dams and reservoirs is. that they block.the upstream :and :downstream movements of fishes. Friant Dam; finished in:.1946,. . cxinrptetely_ prevented a large run of Chinook salmon from reaching their- spawning.: g~ovads._ To make masters worse,. while the reservoir was filling, flow- dawnthe. San: Jdaxluin River was almost completely cut. off, preventing any, salmon that might have.: spza+'aed'in the lower reaches of the rivet from doing sa. Today the capture of a:. saJarton~ in the :upper San- Joaquin River is a rare -event (Moyle, 1970). A similar: bhickage :of the migrations of Colorado squawfish may have been is part.- responsible fdrsheir extinction in California waters. A more subtle effect. of dams is their isolation: ob`thcuupstream areas..If a .stream system located above a Liam should lose its native fish~fauna through natural or man-created disasters, there is na way this system-could be~e®alanized by fishes- from other. nearby stream systems. For examplef California_ raa~ck-are .now absent from the small streams of the. upper San Joaquin River. above _ F13$nt.Dam, with no hope of natural recolonization (Moyle and Nichols, 1974} Risservoirs :are-also- hard on-the native fish fauna because they favor lake:adagted._ introddced _ species over native stream-adapted farms. TFius, in the. Sacramento-San Joaquin system; squawfish and hazdhead tend to disagpeaz from- reservoirs after an initiakfive to ten years of abundance. Reservoirs have, however, benefited some native fishes:-Prickly sculpin and Sacramento sucker are permanently established in a number : oS~entral Valley reservoirs, as are hitch and..tui chub: Sacramento _perclt,-virtually: extinct in their native .habitat, are extremely abundant in a :number of alkaline:. rese~voirsinto which they have been introduced~ Oae:of the main- reasons. for the construction of dams,- reservoirs,: andirrigation- da$fsians is to catch runoff and. send it, via canals, to where it can be used -for irrigation; or for industrial and municipal consumption. This naturally leavesless-water available .for fish downstream from the dam.-One of Lhe most dramatic examples of thesesults of dewatering a stream is the continuous fall in the level of Pyramid Lake, Nlsvada,_ following .diversion of most of the flaw of the Truckee River for irrigation:. TLe sandy delta that was exposed at the mouth of the river due to the declining lake. . le+rek prevented both Lahontan cutthroat trout and cui-ui suckers from spawning n.the _ ..:. _: -