11 



liigrlier slopes are formed of yellowish clays, diti-lied and gullieil hy the rain, 

 with occasional small streams flowing' thi'oug'hf>'orge-like valleys from the level 

 uplands of the country farther west. 



The description thus far g-iven applies to the lower stages of water only. 

 When the river is at Hood the entire l)ottom-land from bluff to bluff is often 

 \\ holly under water, lakes, streams and marshes being then c'orifounded in 

 due unbroken sheet from three to five or six miles across. As the river level 

 Aaries 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 ai'e 

 very commonly two in a year, one beg:inniug in late winter or spring with the 

 melting' of the snows, and the other coming most frequently in -lune or July, 

 as a consequence of the earlj' summer rains. The rise at either or both these 

 periods is oeeasionally so small that no verj- 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, wei'e 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. (". A. Hart, on the entotoology 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 tlie 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 ovei'grown 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 lis 

 a permanent body of water of some little depth, always opening into the 

 river, even at 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. 



