No. 378.] BIOLOGICAL STATIONS IN AMERICA. 401 
The field of operation of the station is, for the present at 
least, the Illinois River and its related waters. Geologists tell 
us that this stream and its bottom lands occupy the bed of an 
ancient river, a former outlet of Lake Michigan. The present 
flood plain is but slightly above the level of the river, and over- 
flows are, therefore, of more than usual extent and frequency. 
The fall of the stream is very slight, about thirty feet in two 
hundred and twenty-five miles, and at times of flood the area 
covered is over seven hundred square miles. Over fifty-six 
square miles in the field of the station’s operations are submerged 
at high water, and of these seventeen represent the river, lakes, 
bayous, and permanent marshes of low-water stages. The 
extreme fluctuation in the river level recorded at Havana is 
eighteen feet, and a rise to sixteen feet above low water is not 
unusual in the spring or early summer. Owing to dams, the 
river at low water is practically a series of slack-water pools. 
The river thus presents a considerable change in conditions 
during the year. Although at high water it is practically a 
unit in environment, as the water recedes a number of distinct 
and characteristic aquatic areas emerge, and are quickly differ- 
entiated by their peculiar fauna and flora. At low water there 
thus lies within easy reach of the station a wide range of situa- 
tions, including the river and its tributary streams, Spoon and 
Quiver Rivers, a shallow ephemeral lake quite free from vege- 
tation, a large impounding lake and bayou without tributaries, 
several spring-fed lakes with different amounts of vegetation, 
and a number of marshes of varying degrees of permanence. 
This extremely varied environment, and the considerable and 
sometimes sudden fluctuations in the water level, add greatly 
to the complexity of the biological problems with which our 
station has to deal.. 
The fertility of the drainage tase of the river, the large 
amount of sewage emptying into the stream, and the rich 
alluvial soil of the bottom lands favor the growth of aquatic 
vegetation. At low water a rank growth of Ceratophyllum 
fairly chokes many of the lakes, and at times even encroaches 
upon the river. Nelumbium and Nymphza, Lemna, Wolffia, 
and Azolla abound, and water-blooms of Euglena, Carteria, 
