407 
greement may perhaps be due to the changes elsewhere, inci- 
dent to rising winter floods. 
SUMMARY. 
Phelps Lake is the richest in plankton of all the localities 
examined by us, averaging 19.65 cm.* per m.*, the mean ofall 
collections, or 22.35 em.*, the mean of the monthly averages. 
This is 8- fold the production in the river and more than twice 
that in any other impounding area examined. We find, how- 
ever, that the lake does not contribute to the river at levels be- 
low 8 ft., and is therefore cut off for a considerable part of the 
time. In the years 1894-1899 inclusive, the days of run-off 
were 14, 0, 71, 139, 145, and 86 days respectively, or an average 
of 76 days. Asa rule the plankton content of the lake waters 
during periods of run-off exceeds that in the channel in varying 
degrees, and the lake by virtue of the impounding function 
serves to increase the plankton content of channel waters. The 
exceptions fall mainly at levels above 10 ft., when the diluent 
action of Spoon River floods affects the production in the lake. 
The high records of production which indicate the great rela- 
tive and absolute fertility of this body of water are in the main 
found during summer and autumn months, when there is no 
discharge and the impounding function is at its maximum. 
This is confirmatory evidence of the effect of impounding when 
the disturbing factor of tributary water is absent and coarse 
vegetation is of little extent. Owing to its small area, its early 
separation from the channel, and its relation to Spoon River at 
high levels, the total contributions from this area are relatively 
small as compared with those from Thompson's and Flag lakes, 
and at times from Quiver Lake, and its relative fertility during 
months of run-off, as compared with these localities, is wont to 
rise above their level of production, especially at stages be- 
tween 8 and 10 ft., when run-off is slight and impounding 
function dominant. Illustrations of this will be found in the 
monthly averages of 1897 and 1898 in April—June, the sea- 
son of greatest run-off, when 4 of the 6 monthly averages 
