288 
distance from the east and west shores respectively. As will be 
seen in Plate XI., this was made after nine weeks of uninter- 
rupted low water, when the river had been standing at 2 ft. 
for some time. The catches were made between 2 and 4 
o’clock p.m. There was no vegetation in the river at this 
point in this season, though both Havana Lake and Quiver 
Chute, to the north (Pl. II.), contained a small amount. The 
discharge from Quiver Creek and Lake makes its way along 
the eastern margin of the river, while that of Spoon River un- 
der these hydrographic conditions hugs the western shore. The 
effect is seen in the turbidity records, the clearer water be- 
ing on the eastern side and the more turbid on the western. 
The following table gives the data of collection. There 
was almost no silt in the catch, and the silt estimates are 
therefore omitted. 
PLANKTON IN CROSS-SECTION OF ILLINOIS RIVER BELOW MOUTH OF SPOON RIVER. 
Tur- Departure from mean 
Num-| Temperature ae Plankton per Plankton under ; 
bidity 3 in per cent. 
ber of (F.) Depth depth m.3, in cm.$ 1sq. meter (omitting Nos. land 2) 
1 Jam] (init moe yan Sige en | = DOG EE | | a 
on Pal a dise Nok Hon meet wer en eee Per m.3 (isGles © 
. oh a ‘O . 
ace tom visible | ume in per cent. ume a per cent. sq.m 
1 Tes} 76 66 35 2.40 —60 1.58 —83 
2 71.5 70 1.06 33 3.88 =35 4.11 —57 
3 70.5 70 1.42 35 5.40 —10 7.67 —20 —20 =32 
4 70.5 70 1.58 30 6.32 +5 9.99) +4 — 6 —12 
7 ae uy ne au 5.60 aot 8.85 —8 —17 —22 
70.5 1.6 +2 7.64 +27 12.84 +34 +14 14 
7 70 5 70 1.83 225 8.20 +37 15 00 +56 +22 13 
8 70.5 70 1.88 +22, 7.40 +23 13.91 +45 +10 +23 
9 71 70.5 1.83 -20 6.84 +14 12.52} +30 + 2 +11 
10 71.2 71 1.72 aie 6.28 = 5 9.55 1 6 15 
Average | 6.00 |=:22.3 9.60] 338 | 121 | 20.2 
The results of this test are confirmatory of the thesis here 
maintained, namely, that the distribution of the plankton in a 
stream does not differ in the main from that thus far observed 
in lakes in the matter of variations in the plankton content 
(volumetric) in different localities. The average + depar- 
tures from the mean volume, computed per m.*° and under 1 
sq. m., in these ten catches are 22.3 and 33.8 respectively, as 
compared with 31.8 and 28.8 for Lake St. Clair and 5.52 for 
the German lakes examined by Apstein. 
