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' • TABLE 2. Watgr quulit~c o/ the San Ralae~,and tributaries 1977.. _Characteristio Station Water Total Dissolved Turbidity Con- Hardness pH Phos hate and temp (C) alkalinity oxygen (FT[I's) ductivity (mg/1) (mg/i) month mg/1 (mg/1) (µohms) - species is probably due to differences in habitat between the upper and lower sections of the river. The lower San Rafael River consisted of wide, shallow stretches, with uniform sand bottoms. The few ools were small and shallow. Introduced species appear to be bette~- p Habitat in the upper ironment f hi i i 1 Apr 4 350 10.5 500 2450 1570 9 1 - . env s type o ve n t able to surv stations was more diverse; pools were deep and long, cover was . Tun 19 230 5.8 5 - 1600 8.4 0.2 more frequent, and substrate was .more variable. Areas with more Oct 15 , 150 4.7 - 3700 - - - diverse habitat are apparently more suitable for native fish. 2 Diversion of water for irrigation probably is the major cause of Jun 29 320 - 65 - 1750 8.3 0.1 habitat differences between the upper and lower sections of the river. Oct 20 200 6.5 - 3800 - - - Small reservoirs control streamflow in each of the tributaries. How- 3 Jun 24 230 5.0 5 - 1490 8.4 0.2 ever, much of this water is diverted before it reaches the lower Oct i8 230 1.0 - 5500 - - - San Rafael River. 4 ~ The combined effects of drought and diversion of water for irri- Apr i 370 13 50 2250 ]450 9.] gation played an important role in distribution of fish in the San Jun 31 250 6.3 l0 - 1590 7.8 p,4 Rafael River during 1977. In October, these ,two .factors created Oct 14 200 7.5 - 6000 - - - extreme conditions in lower sections of the river (stations 1, 2, and g Mar 10.5 335 10.5 20 6000 1920 s.8 0.1 3), when fish were strar>,ded. in isolated pools along the otherwise Tun 23 230 5.3 65 - 1440 8.4 - dry stream bed. Desert fishes are often able to survive for extended Oct 20 200 7.0 - 5250 - - - periods in these small pools, even with the additional stress of 6 disease, parasitism, and predation (Deacon and Minckley 1974). Jun 23 290 5.0 30 - 1520 8.4 0.2 Low dissolved oxygen may also play a role in the decimation of the Oct is 350 5.0. - 7000 - - - 7 ~ crowded fish, particularly when organic material creates a high Mar 7 390 9.8 30 5500 1785 8.6 0 biochemical oxygen demand (Larimore et al. 1959). Dissolved oxy- Jun 27 280 6.3 2 - 1450 8.4 0.4 gen in pools at station 1 ranged from 1 mg/1 to 4.8 mg/1. Pools Oct 12 510 7.0 - 5960 - - - h were also inhabited by beaver had low oxygen-levels due to hi 8 c w decomposing fecal material and other organic matter, and contained Mar 8 385 7.3 25 7800 2045 8.6 0.1 Jun 16 330 5 5 30 is o 8 few fish. More fish were found in pools where oxygen levels were . - i .7 0.2 9 higher. Jun 24 340 3.0 45 - 1590 8.4 0.2 Fish repopulation of decimated riverine areas after dewatering Oct 18 300 7.4 - - - 7.1 - --- (Larimore et al. 1959) or pollution-caused fish kills (Krumholz an Minckley 1964, Gunning and Berra 1969, Olmstead and Cloutman from 7.1 to 9.1. Water temperature was 1 C in March and rose to 31 1974, Phinney 1975) is often swift. We observed repopulation of Fer- C by June. ron Creek (station 9) in October after dewatering in June, In June, streamflow was reduced such that fish were confined to a shallow DISCUSSION. The distribution of native and introduced fish in pool. However, streamflow had resumed by October and the fish the San Rafael River reflects a pattern that has become common in population was larger and more diverse (Table 1). the West. Several investigators have reported introduced species re- The collection of ayoung-of-the-year Colorado squawfish repre- placing native fish in lower reaches of rivers and streams (Deacon Bents the first record of the species in the San Rafael River. Colorado et al. ]965, Deacon and Bradley 1972, Cross 1976). squawfish are considered to be large-river ,fish (Minckley ]973), In the San Rafael River, the segregation of native and introduced and have only rarely been reported in rivers other than the -large 47 46 ~ i