314 



curve to the left, giving a left-handed skewness to the curve. 

 Aside from this depression due to flood there is a general de- 

 cline in production as levels fall, the pulse closing on the 21st 

 with a minimum of .28 cm.^ This decline in production is at- 

 tended by a steady rise in nitrates, organic nitrogen, and free 

 ammonia (PI. XLIIL), and thus in the presence of increasing 

 nutriment, as well as growing hydrographic stability — that is 

 lower river levels. The two summits of production in this pulse 

 coincide with temperature pulses. The plankton and tempera- 

 ture pulses are alike set off by flood waters, and the causal 

 nexus may lie between plankton and flood rather than between 

 plankton and temperature. The location of the flood also has 

 the effect of lowermg the average production of the month to 

 1.12 cm.-' — the lowest average on record, excepting only 1898 

 (.91), also a year of much disturbance. 



From this point the remaining collections of 1896 are too 

 infrequent to delineate or even to suggest recurrent volumetric 

 pulses. They are also insufficient to adequately trace the results 

 of hydrographic changes. Diminished production at the time 

 of rapid decline of temperature is apparent late in September. 

 Increased production follows declining flood and stable tem- 

 peratures and a downward movement of nitrates (PI. XLIII.) 

 in October, and the phenomenon is apparently again repeated 

 in December, though the volumes do not equal those of the 

 preceding year. 



As a whole, 1896 was a year of but slight plankton produc- 

 tion, averaging only 1.16 (average of all catches) or 1.05 (aver- 

 age of monthly averages) cm.'' per m.^ This is only a half or 

 a third that of other years in our records (see table on p. 292). 

 The silt, on the other hand, is more abundant than in any other 

 year, averaging 2.55 cm.^ per m.^ to .28, .72, 1.91, and 2.11 re- 

 spectively for 1894, 1895, 1897, and 1898. The total movement 

 in levels for this year at Copperas Creek is 58.16 ft., an excess of 

 that in all other years but 1898 of 4 to over 40 per cent. (Ta- 

 ble I.). From this fact, and from the evidence accumulated in 

 the detailed discussion, it is apparent that the oft-recurrent 



