KEOKIJK DAM 
119 
was unavoidably prevented from continuing the investigation, and the data that he 
obtained in 1914 have remained unpublished. It is desirable to include here an 
abstract of his observations as prepared by him (next three following sections). The 
later study of the plankton of the upper Mississippi in 1921 by Doctor Galtsoff (1924) 
included Lake Keokuk, although he could not, in the circumstances, work this region 
intensively. In comparing the results of the two studies it must be remembered that 
there is much reason for believing that a new lake is richest in its first years. 
ENTOMOSTRACA 
1. In the earlier portion of the season (July) the Entomostraca in the main 
channel of the Mississippi River and in Lake Keokuk increases in numbers with 
approach to the dam at Keokuk, as illustrated by the following figures showing the 
approximate number of Entomostraca per cubic yard of water: At Oquawka, 55 
miles from the dam and above its influence, 50; at Burlington, 52 miles from the dam, 
100; at Nauvoo, 12 miles from the dam, 150 (over submerged land, 250, and in the 
protection of weeds, 450); at Keokuk, 2 miles from the dam, 1,500. The increase is 
attributed to the more favorable conditions accompanying the slackening current, 
such as the partial loss of sediment in suspension and the greater time allowed both 
for development of food materials and the multiplication of the Entomostraca. In 
weeds these favorable conditions are increased and, in addition, there is furnished 
shelter and fundamental food. 
2. The tributaries of Lake Keokuk and the Mississippi River in July show an 
Entomostracan population greatly exceeding that of the main river and lake at this 
time. The numbers apparently vary with conditions and somewhat as follows: 
(a) The larger tributaries or arms, without current and with relatively small open- 
ings upon the river or lake, may have 50,000 Entomostraca per cubic yard; (b) 
tributaries or arms without current, whose openings upon the river or lake are rela- 
tively larger, yield about 2,500 per cubic yard; (c) tributaries through which there is 
current, though scarcely appreciable, possessed Entomostraca at the upper limit of 
backwater in the number of about 5,000, increasing downstream to about 20,000 
until coming within the direct influence of water from the main channel. 
3. The river and lake in August showed the following changes in Entomostracan 
content, generally in the direction of substantial increase in population, as may be 
seen by comparing the following approximate figures for August with those pre- 
viously given for July: Above the prism of the dam (Oquawka), in the low water 
then prevailing, 35; within the prism of the dam at Burlington, 100; at Nauvoo, 
1,500 in the main channel (20,000 to 30,000 over submerged land and 150,000 among 
weeds); at Keokuk, 25,000. The increases are attributed to the same factors that 
operated in July. 
4. In August the tributaries showed differences, but not the same kind of dif- 
ferences as in July : (a) In tributaries without current and shut off from the lake by 
relatively narrow openings or having little means of preventing stagnation the Ento- 
mostraca had decreased to 25,000 per cubic yard; ( b ) in tributaries or arms of the lake 
not subject to stagnation the Entomostracan population had held its own or increased 
to 50,000 and 100,000 per cubic yard; (c) in the river and the tributaries with cur- 
rent the Entomostraca maintained or increased their abundance, the numbers 
running from 5,000 to 30,000 per yard but being always less in the upper limit of 
backwater. 
