NOTES ON THE FISHES OF THE STREAMS FLOWING INTO SAN 



FRANCISCO BAY. 



By John Otterbein Snyder, 

 Assistant Professor of Zoology, Leland Stanford Junior University. 



The territory drained by the streams flowing into San Francisco Bay 

 comprises a catchment basin which is partly bounded b}^ mountain 

 ranges or considerable height. It is thus sharpl}^ separated on the 

 east from the San Joaquin Valley, and on the west from a much more 

 restricted area drained by a series of small streams flowing directly to 

 the ocean. On the south a comparatively low, though perfectly dis- 

 tinct, watershed divides it from the valley of the Pajaro River. All 

 of the streams connected with the ba}^ are to be considered as belong- 

 ing to a single s3'Stem, none apparently having remained isolated for 

 any considerable period of time. Complete isolation is prevented by 

 an occasional intermingling of the waters of two or more streams near 

 their mouths, and also by a reduction of the salinity of the water of the 

 bay during periods of excessive rainfall, the surface at such times occa- 

 sionall}^ becoming quite fresh. 



Most of the streams of this basin converge toward the southern end 

 of the bay, which is there bordered by extensive salicornia marshes. 

 The constant wash of the tides has cut into the surface of these 

 marshes a network of sloughs, to some of which the water from the 

 creeks eventually finds its way. Before reaching the sloughs, however, 

 this water often spreads out, forming large ponds. The union of two or 

 more of these temporary ponds, the shifting of a creek channel caused 

 by some obstruction, the change in the direction of a slough, or a com- 

 bination of these conditions ma}^ form between two streams a continu- 

 ous passage well adapted for the migration of fresh-water fishes.^ 

 Such a union of two creeks has actually been observed, one of them as 

 a result having become stocked with an additional species. A dense 

 growth of willows recently deflected San Francisquito Creek to the 



(1 Such conditions are possible only during the height of the rainy season. On the approach of the 

 dry season all the streams of the region rapidly shrink, both in volume and length, only one of them. 

 Coyote Creek, discharging water into the bay during the entire summer. Much of its bed is dry, how- 

 ever, for part of the year, the water sinking soon after leaving the mountains, and appearing again 

 about 2 miles above its mouth. 



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