344 
to some extent with the channel water of Spoon River, and the 
prolonged drouth of the autumn cut down the run-off and re- 
duced the stream to a series of slack-water pools, in which, 
owing to the reduction in current, there was time enough for 
an abundant plankton to develop. 
The explanation of the contrast between the plankton con- 
tent of this stream on February 3 (.002) and 26 (.092) is to 
be found in the hydrographic conditions. The tributary shares 
the rise in production seen in channel waters (.03 to .05). The 
rising flood of the 26th forces the impounded backwaters away 
from the channel, and in their downward movement some of 
them get into Spoon River channel in the overflowed bottom- 
lands above the point at which our collection was made. 
Thompson’s Lake waters contained considerable plankton (.39) 
at this season, and it seems probable that some of its richer 
waters may have entered and (PI. II.) enriched Spoon River 
channel plankton at this stage of river. Under such circum- 
stances we find the tributary with a richer plankton (.092) than 
the storm-filled channel (.05)—an exceptional occurrence in the 
history of the two streams. The very slight production (.007) 
on March 22 is due to the fact that Spoon River itself at this 
time was rising rapidly, turbid (2 cm.) with silt, and invading 
rather than receiving contributions from the impounded back- 
waters through which it rushed to the channel. The collection 
of April 27 was also in flood waters (turbidity 5 em., silt 4.75 
em.’), which are in part responsible for the check in the flood 
decline at that time (Pl. XXIII.). This held back contribu- 
tions from connecting and impounded backwaters, and the 
plankton content is low (.048), while that in the main stream 
(5.11) shows no such flood reduction. In the collection of May 25 
(.44) we find the tributary waters as well as the main channel 
exhibiting a vernal rise in production, though its amplitude is 
13-fold greater in the latter. The lower river level (8 ft.) then 
prevalent precludes the possibility of any considerable contri- 
butions from impounding areas, though accessions in small vol- 
ume are not improbable, On June 28 the silt-laden storm-water 
