40 



tidal influence extended up Sacramento River as far as Sacramento. The tidal range 

 was there in excess of 2 feet and extreme low water fell at times below the zero of 

 the Sacramento gage, which is 0.48 foot below mean sea-level. Under the effect of 

 hydraulic mining the bottom of the river was raised by sand deposits and the rising 

 bottom forced up the water surface of the river so that the lowest low waters of the 

 years 1895-1899 a\'eraged about 8 feet higher than those which pre\ailed originally. 



In recent years the bed of the Sacramento River has again been lowered to near 

 its original position. The principal causes which ha\-e contrilnited to this depression 

 of the river bed are the cessation of hydraulic mining on a large scale; the restraining 

 of mining debris under Go\'ernment direction at or near the mines; the extensive 

 dredging operations carried on in the rivers to secure material for river le\-ees, and 

 to enlarge the lower reaches of the river; and the scouring action of the river, which 

 is better confined between the ri\-er levees in recent years than theretofore. 



The changes which ha\'e occurred in the low water elevations at Sacramento 

 are well illustrated by the following figures from gage readings at Sacramento: 



TABLE 2 



LOW WATER AT SACRAMENTO 

 Gage Readinx.s at Lowest Stage of Tide 



Average of A\'erage of 



Period annual lowest Period annual lowest 



gage readings gage readings 



1849-53 0.3 1889-93 7.7 



1854-58 1.0 1894-98 7.9 



1859-61 1.6 1899-03 7.2 



1862-73 no record 1904-08 6.8 



1874-78 5.3 1909-13 4.4 



1879-83 6.7 1914-18 3.2 



1884-88 7.3 1919-20 0.4 



As the destructive work of the teredo in the upper bay region apparenth' did not 

 become serious until within the two or three )'ears preceding 1920, it is of particular 

 interest to note the fact that the climatic year 1917-18 was a year of very light rain- 

 fall and small run-off (see fig. 17); that the rainfall of 1918-1919 in aggregate amount 

 was about normal at San Francisco and Sacramento but that it was so distributed in 

 time that it produced materially less than normal run-off from the watersheds of the 

 Sacramento and San Joaquin Ri\'ers, and that both rainfall and run-off for 1919-1920 

 were less than 50 per cent of normal. 



During the low water period of these years the irrigation draft upon the Sacra- 

 mento and San Joaquin Ri\'ers was increasingly heavy, due largely to rice growing in 

 the Sacramento Valley, and there was a period of several months of the year 1920 

 during which it seems to ha\'e been a fact that more water was used for irrigation on 

 the delta lands of these rivers than was supplied to the delta channels b^• the two 

 rivers. This, together with the maintenance of a flow in the lower river, was of course 

 possible only by reason of subsurface return of a portion of such irrigation water to 

 the river. In such circumstances some of the bay water which entered the mouths of 

 the ri\-ers on each flood tide was retained in the river channels and caused bay water, 

 under the influence of tidal action, to get up stream at high tide to points in the 

 Sacramento River somewhat above Isleton and in the San Joaquin River to points 

 probably above the mouth of Middle River. The salinity records of the State Water 



