12 University of California Publications in Zoology [Vol.22 



At any rate, it seems that one explanation of the apparent numerical 

 deficiency in plankton production as compared with the Illinois may 

 be that there is less impounded water ready to develop and discharge 

 plankton, and that there are no frequently occurring minor floods to 

 wash out these areas. It should be stated, however, that 1913 was an un- 

 usually dry year; hence the flood conditions were not typical for this 

 region. 



Effect of the Mountains 

 The mountains probably have just as much influence, on plankton 

 in the river as does the character of the bottom lands. The Coast 

 Range has no very extensive influence. In the main it is rather 

 against plankton production. The slopes are steep and the run-off 

 torrential during the heavy rains. There are not many natural reser- 

 voirs such as either swamps or lakes, and the surface water is soon 

 lost. Consequently very little plankton is contributed to the San 

 Joaquin from these western tributaries. In fact most of them are 

 dry through a large part of the year and the water they contribute in 

 time of flood is so full of silt as to hamper rather than hinder plank- 

 ton production in the main river. 



Temperature 

 The Sierra Nevada seems to affect production in two rather im- 

 portant ways. First, temperature in their run-off is rather low for 

 either quantity or variety of plankton in the higher areas during most 

 of the year. On the other hand, there are few places for impound- 

 ing the water in the lower areas and the streams move too rapidly for 

 much development even on the comparatively low gradients of the 

 foothills. Hence there is no very great contribution of plankton from 

 any tributary. Second, the snows, glaciers, forests, lakes and swamps 

 of the higher region all together constitute an enormous series of res- 

 ervoirs which hold much of the water in check, not only greatly pro- 

 longing the flood season, but giving a remarkably even distribution of 

 flood water over a period of weeks or even months. The effect of this 

 is evidently twofold, inasmuch as the volume of water hastens the 

 flow of the river and so may retard production, while at the same 

 time it keeps many of the sloughs and swamps sufficiently filled to 

 maintain a rapid plankton output. This period of flood from the 

 Sierra often overlaps the period of floods in the valley due to the 

 winter rains. In other cases it follows or is continuous with the same. 

 Hence there are a few rises and subsidences of the floods, almost every 

 year, some of which are effective in clearing out the sloughs and giving 



