449 
months of flood disturbance falls 79 per cent. below the mean 
for that season, we find in the 10 months of depression in 
Thompson’s Lake a falling off of only 46 per cent. The differ- 
ence is due to the greater proportion of creek and spring water 
of recent origin in the former, and to the greater reservoir ca- 
pacity and consequent longer impounding, as a rule, of the 
sewage-laden channel waters which predominate in the latter 
backwater. 
Production in Phelps Lake is 37.34 cm.’ per m.*, or, if all 
collections are averaged, 36.31—67 or 84 per cent. above the 
means for all years. In keeping with these facts we find that 
the monthly averages equal or exceed the means for their 
months in 7 of the 10 months of record, the greatest excess oc- 
curring during the period of complete isolation of the lake. 
1899. 
(Pl. XIIL, XXIV., XXIX., XXXIX., XLII.) 
This year is represented by 13 collections in the Illinois 
and 3 in Spoon River, and by 7 each in Quiver and Thompson’s 
lakes and 6 in Phelps Lake, all in the first three months of the 
year. This was a period of a slow rise of the river in January 
to bank height, with an equally slow decline in the next month 
followed by an abrupt and well-sustained March flood. The 
data of comparative production are brought together in the 
following table. 
PLANKTON PRODUCTION IN I899. 
l | Mean of 
Station | Jan. | Feb. | March |} monthly 
| | averages 
MNfHOISMRIVE RS mec ace ese eeces | anaes | =| «18? | =F) 8x eg BS \Sr) siiey 
SPOOMURIVE Faye cio Aeeiciejstorainis stele eirteis stemtc =| .005;—| -oor|—j .026/\—| .orr 
Quiver Wake vn ccccs ssl cscs cieiaisisience (Su) see |S regas? |letl ne 84) aes 
ahompsonismuaker yan acwe sce ceedeecres. ve —| 1.64 | 1.59 |—| .2! —| 1.15 
Phelps! Wake t con Seton. cistesw crn clioes patel 4.69 paeFOnl 1.82 |—| 3.74 
*The + and — signs for this column refer to the relation of these averages to the 
mean of all January—March collections; otherwise they are used as heretofore. 
The average production in these winter months in the river 
is .42 cm.* per m.*, or, if all collections are averaged, .41 cm.* 
