313 



progresses irregularly to a slight maximum of 2.24 ou the 

 20th during a very rapid rise caused by a Spoon River flood 

 (PL II.) which, while not invading the stream above its mouth 

 to any great extent, held back the water from the upper river. 

 The greater part of the rise in the latter part of the month was 

 due to the access of water below the plankton station or in re- 

 mote headwaters, and is thus a reflection of the rise in the lower 

 river or distant tributaries. The freedom from silt apparent 

 in the catches bears testimony to this fact. The pulse reaches 

 its culmination and declines in this rising flood. 



The average production for July in 1896 (1.44 cm. 3 ) is less 

 than that in any other year save 1898, and that, too, in what 

 seem to be favorable hydrographic conditions. The sharp de- 

 cline in nitrates (PI. XLIII.) from 2.8 to .4 parts per million 

 may be a factor in the small production. 



The August pulse has a duration of 27 days, — from July 25 

 'to August 21, — with a maximum amplitude of 3.90 cm. 3 on 

 July 30. Its mean falls on the 5th, 16 days after that of the 

 preceding pulse. This was predominantly a month of falling 

 levels. The culmination of the rise at 8.6 in the first six days 

 is followed by a steady decline reaching 5.8 on the 31st, broken 

 only by the slight interruption in the middle of the month. 

 The total movement is 3.7 ft., and the total at Copperas Creek 

 (4.60) is somewhat above the average (4.06). The disastrous 

 effect of the local floods at the culmination of the rise which 

 flushed out the rising plankton pulse is apparent in the de- 

 cline from 3.90 cm. 3 on July 30 to .40 on Aug. 1, and in the in- 

 troduction and continuance of a considerable volume of silt. 

 The rapid recovery of the plankton is seen in the rise from 

 a minimum of .26, .48, and .32 on Aug. 3-5, to 1.08, 2.40, and 

 2.60 on Aug. 6-8. These data suggest the intercalation of 

 barren flood waters of recent and local origin in the course of 

 channel waters bearing a much more abundant plankton, and 

 the apparent result is a cleft in the otherwise somewhat sym- 

 metrical curve of production of this August pulse. It also re- 

 sults in an apparent shifting of the node and means of the 



