307 



exceed, if indeed equal, that of years of more normal hydro- 



81 ' aph - ,8%. 



(Tables III., X. ; PI. X., LI.) 



There were 76 collections made in this year, of which 69 

 are prior to Sept. 1 and are, moreover, at intervals brief enough 

 to enable us to trace the curve of plankton production with 

 some degree of accuracy. In the last four months the fort- 

 nightly interval is too great to permit more than conjecture as 

 to the probable course of the plankton curve. 



The collections prior to May 22 were all the result of com- 

 bining 4 to 9 repeated vertical hauls of the net. Subsequent to 

 that date they were made with the plankton pump. This, as is 

 shown on page 165, was a year marked by recurrent floods, 

 which bring the average height for the year up to 6.98 ft. in a 

 year of less than average rainfall. This is almost twice the 

 average height (3.61) of the preceding year. Since the flood 

 did not in most cases reach bank height, the overflows were 

 not extensive and did not occur during periods of large plank- 

 ton production (PI. X.). The distribution of the collections 

 with reference to the floods is such that we have again in this 

 year the opportunity to test the effect of the access of flood wa- 

 ter upon the curve of plankton production at all seasons of the 

 year but the autumn months. In this year the vernal rise in 

 temperature occurred abruptly in the middle of April, and 

 the autumn decline began quite early but progressed slowly. 

 Summer temperatures were also lowered somewhat by access 

 of flood water. 



The plankton of January, February, and March (PI. X.) 

 forms so small a portion of the total catches that its quantita- 

 tive changes are swamped by the probable error of silt estima- 

 tion, and are apparently of such slight extent that their signifi- 

 cance cannot be detected. The amount of silt carried is very 

 large, doubling or trebling in quantity on rising floods, and 

 reaching a maximum of 14.77 cm. 3 per m. 3 on the crest of the 

 March freshet. No recurrent pulses appear in the volumetric 



