(July 15>), salmon soon disappeared from tow-net catches. In 19h9s, 

 although 75 degrees was reached in the middle of June, the temperature 

 dropped immediately, and July 10 is taken as the reference. Again sal- 

 mon disappeared very shortly from the catches. This suggests that water 

 temperatures are an important factor in regulating the exodus of salmon c 

 from the Delta) and that once an upper temperature limit is reached, 

 movement seaward is rapid. 



Discussion 



The association of seaward migration of immature king salmon mig- 

 rants with flood flows is historical in its annual recurrence and has 

 been demonstrated in the past and present studies of the early life 

 history of salmon in Sacramento and San Joaquin Rivers. During 

 the course of this investigation the initial appearance of salmon fry 

 from Sacramento River sources was observed at the time of the seasonal 

 increase in runoff. Rutter (1902), Hatton (I9I1O), and Hatton and Clark 

 (19u2) obtained similar results in their studies „ This phenomenon was 

 also observed by Hatton in the San Joaquin drainage during years of 

 normal rainfall and prior to storage of runoff waters at Friant Reservior. 

 Sub-normal water supplies in California's Central Valley during the period 

 of this study affected downstream migration of juvenile salmon from the two 

 principal sources, particularly in the San Joaquin drainage. The escapement 

 from the Sacramento River drainage was least affected, since the runoff, ai- 

 through less in each of the three years than the fifty-year normal, still 

 consisted of much uncontrolled water flowing in its historical pattern. 

 Several factors altered the downstream escapement of salmon migrants from 

 the San Joaquin drainage. The principal one was the storing of San Joaquin 

 waters behind Friant Dam (Millerton Lake), thereby limiting runoff. Severe 

 drought conditions in the San Joaquin Valley further affected the seaward 

 migration and high water temperatures were undoubtedly effective in re- 

 stricting downstream movement. 



Upon reaching the peripheral limits of the Delta, the Sacramento 

 migrants were apportioned with the runoff into its diverging channels. 

 The migrants dispersed down the main stem of the river and the three sloughs 

 off the river, Sutter, Steamboat, and Georgiana. Georgiana Slough is of 

 particular importance because it is the chief connecting channel through 

 which Sacramento water and migrants flow into the San Joaquin part of the 

 Delta. The main body of the Sacramento River migration entering the Delta 

 moves more or less directly seaward with a significant segment diverting 

 into the San Joaquin Delta. This investigation has demonstrated that upon 

 entering the San Joaquin Delta with its larger tidal basin the transferred 

 migrants tend to remain within it for a period of time and a proportionate 

 number disperse throughout the Delta with the Sacramento River water (see 

 Figs. 3 and it). • 



20 



