407 



interchange between lake and river. The average height for 

 this year is 8.02 ft., the highest during our years of record. 

 From Jan. 22 to July 15 river levels were above 6 ft., and a 

 constant inflow of impounded water from bottom-lands above 

 the lake, or through the slough when overflow ceased, continued 

 with impounding in the lake and subsequent discharge from its 

 southern end to the channel. The same conditions again 

 prevailed from Oct. 30 till the end of the year, with an inter- 

 ruption of 6 days in December. During the remaining parts of 

 the year there was a constant wavering in levels which fa- 

 vored frequent — in fact, no less than 21 — reversals in the direc- 

 tion of flow in the slough connecting the lake with the river. 

 During the 134 days of low water there were 56 of falling levels 

 in which the lake was discharging its plankton-laden water 

 through the slough to the river, making a total of 287 in which 

 it contributes to channel plankton to 78 in which, owing to low 

 levels, it merely receives an inflow from the river. Moreover, 

 the periods of greatest plankton production in the lake, during 

 the vernal pulse, occur at times when the run-off from the lake 

 is at its height, so that in this year all the hydrographic factors 

 combine with the distribution of the plankton production to 

 render this reservoir lake a feeder of the channel plankton. 

 Though the differences in the plankton content are such that the 

 actual enrichment per cubic meter may be less than in other 

 years, the total run-off of plankton into the channel must com- 

 pare favorably with that in any other year of our operations. 



The comparison of coincident collections shows in all cases 

 but three, a greater plankton content in the lake than in 

 river. The first of these is on April 5, at the height of the 

 spring flood, when a considerable current sweeps through 

 Thompson's Lake and shortens the period of impounding, and 

 thus reduces the time for the development of the plankton. 

 The second instance is on June 21, on the decline of the acces- 

 sory vernal pulse, which reaches a lower level in the lake 

 (2.47) than in the river (2.88). This is one phase of a not un- 

 common phenomenon in the plankton pulses of the backwaters. 



