34 



change under the ever \'arying forces of nature and yet so Httle altered in the liaU' 

 century during which it has been repeatedly surveyed that no characteristic or pre- 

 dominating tendency to change has yet been discovered. 



The watershed area tributary to the Golden Gate is about 62,000 square miles. 

 Of this area 2014 square miles are tributary to the main or south arm of the bay; 

 964 square miles are directly tributary to San Pablo Bay; 557 square miles are in the 

 watersheds of small streams discharging into Suisun Bay; 58,000 square miles are 

 embraced within the drainage basins of the Sacramento and San Joaquin Rivers, 

 which discharge into the upper end of Suisun Bay; and 460 square miles, as above 

 noted, are bay water surface. 



TIDES AND THE TIDAL PRISM IN RELATION TO 

 FRESH WATER ACCESSION AND SALINITY 



Concerning the tides in San Francisco Bay the late Prof. Geo. Davidson, in the 

 "Pacific Coast Pilot," says: 



"From the lower low water (low water large) the tide rises for about 

 7V4 hours, say 4.4 feet, to the smaller of the two high tides (high water small) ; 

 then falls 1.4 feet in less than 4)2 hours to the 'low water small' which is 

 higher than the preceding low water; then rises, say 2.9 feet in 6^4 hours to 

 the higher high water or 'high water large'; it then falls again 5.8 feet in 

 over 7 hours to the lower low water, or 'low water large.' 



"Instead of the above figures, the fall from high water small or 'half 

 tide,' to the 'low water small,' may range from .S^ 2 feet at one position of the 

 moon to 0.3 feet at another; in the latter case there will be apparently a long 

 stand of about 5 hours. . . . 



"The average difference of the higher high and lower low waters of the 

 same day is 5.2 feet, with a greatest observed range noted for February 8, 

 1876, of 9.93 feet." 



Careful studies were made by the late Dr. G. K. Gilbert of the tidal movement 

 in San Francisco, San Palilo and Suisun Bays,* as the result of which the following 

 may be noted : 



The average volumes of great tropic ebb currents, as dependent on the effective 

 tidal prisms of bays, other liodies of open water and marsh tracts, are estimated by 

 Dr. Gilbert as follows: 



At Golden Gate, 91,948,000,000 cubic feet or 2,110,000 acre feet. 

 At San Pablo Narrows, 39,800,000,000 cubic feet or 910,000 acre feet. 

 At Carquinez Strait, 20,580,000,000 cubic feet or 470,000 acre feet. 

 He gives the following as the effective ranges of tide (great tropics) : 

 In Golden Gate, 6.2 feet. 

 In San Pablo Bay, 6.2 feet. 

 In Carquinez Strait, 7.2 feet. 

 In Suisun Bay, 7.3 feet. 

 The Sacramento and San Joaquin Rivers, the drainways of the Great Central 

 Valley of California, meet at Collinsville at the head of Suisun Bay. This Bay is of 

 particular interest at this time because it is, at low stages of the rivers, the principal 

 mixing pool of river and ocean waters. Its tidal area under natural conditions was 

 larger than now by about 100 square miles of marsh lands, formerly submerged at 



*See Professional Paper No. 105, U. S. Geological Survey, "Hydraulic Mining Debris in the Sierra 

 Nevada," by Grove Karl Gilbert. 



