1] 
Dhigher slopes are formed of yellowish clays, ditched and gullied by the rain, 
with occasional small streams flowing through gorge-like valleys from the level 
uplands of the country farther west. 
The description thus far given applies to the lower stages of water only. 
_ When the river is at flood the entire bottom-land from bluff to bluff is often 
wholly under water, lakes, streams and marshes being then confounded in 
one unbroken sheet from three to five or six miles across. As the river level 
varies some eighteen feet between high and low water mark, it may reach in 
its deepest part a depth of nearly thirty feet. These periods of inundation are 
very commonly two in a year, one beginning in late winter or spring with the 
melting of the snows, and the other coming most frequently in June or July, 
as a consequence of the early summer rains. The rise at either or both these 
periods is occasionally so small that no very marked effect on the biology of 
the river is produced. It was, in fact, fortunate for our operations that the 
first two years of our occupancy of the Station were marked by this compara- 
tive uniformity in the river level. Observations and collections made at this 
time have given us a fairly steady biological base line, by comparison with 
which variations in other years may be detected, due to extensive overflow 
and subsequent recession of the waters. 
The plan and purpose of our work was such as to make it necessary that 
we should choose a number of regular stations—called substations in our re- 
ports—at which collections should be made and observations placed on record 
at regular periods for the entire year, and one year after another. These sub- 
stations, thirteen in number, were chosen to represent the greatest variety of 
biological situations which the territory within our reach would permit. They 
have been sufficiently characterized in the introductory part to a report by 
the Station Entomologist, Mr. C. A. Hart, on the entomology of the Illinois 
River and adjacent waters, published in the Bulletin of the State Laboratory 
of Natural History in 1895. It may be said in general that the substations 
chosen represent the springy bank and sandy margin of Quiver Lake and of 
the river itself in both swift and sluggish water, the opposite mud bank of 
river and lake, shallow mud flats overgrown with water weeds, the bed of 
‘river and lake in the deepest water occurring, and three forms of bottom-land 
lakes, together with a fourth occasionally visited. Thompson’s Lake gives us 
a permanent body of water of some little depth, always “opening into the 
river, evenat its lowest stage, but contrasting with Quiver Lake in the fact that 
this opening is long and tortuous, while in the latter it is half as broad as 
‘the lake itself. Matanzas Lake, on the eastern side of the river but below 
the town, is substantially intermediate in character between these two. Like 
‘Quiver Lake, it has a high, wooded, sandy eastern shore and low forest-covered 
‘mud banks on the west, with an inlet at the head, which is, however, smaller 
‘than Quiver Creek. The flow of spring water from the sand is much more 
‘abundant. Like Thompson’s Lake, its outlet is narrow, but it is very short. 
‘This lake commonly has more vegetation than Thompson’s and less than 
‘Quiver Lake. In Flag Lake we have little more than a fairly permanent 
‘swamp, subject, indeed, in extraordinary years to be dried out completely, 
