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northern part. This is due to the greater precipitation during 

 the winter months in the southern part, the rainfall during the 

 remainder of the year being practically the same in all parts 

 of the state. 



The Run-off. — Eighty per cent, of the Illinois River water- 

 shed lies in two principal basins, each with peculiar climatic 

 and topographical conditions. These basins meet at Havana, 

 and consequently the conditions in the northern one constitute 

 a prominent feature in the environment of the plankton at 

 that point. As the two basins lie in different storm-tracks 

 their rain-floods do not always coincide. The southern basin 

 usually parts with its snow several days sooner in the spring, 

 and more promptly, than the northern, since its latitude is 

 more uniform, and, relatively, its floods are larger. The 

 climatic conditions of the southern basin are manifest at Ha- 

 vana principally in the backwater from floods in the lower 

 river, which check the current and delay the run-off from the 

 northern basin. 



The subject of fluctuations in the volume of water in the 

 river is one of fundamental importance in our plankton work. 

 With rising water comes a decided increase in the current and 

 a rapid displacement of the sluggish waters teeming with mi- 

 nute life by a turbid flood to a large extent devoid of life. 

 With the overflow of the bottom-lands the subordinate 

 lagoons, lakes, and marshes are for the time being obliterated 

 from the landscape, and their peculiar fauna and flora mingled, 

 merged, and swept away with the flood. With the decline of 

 the flood, the unusual prolongation of which in the Illinois 

 River has been already alluded to, a great variety of conditions 

 of current, temperature, depth, light, and vegetation are 

 afforded, the most of which favor the development and diversi- 

 fication of the plankton. The large amount of silt — composed 

 of a wide variety of substances, from an impalpable earthy 

 material to coarse sand and the comminuted debris of vegeta- 

 tion — introduced by flood water presents a most perplexing 

 problem in our quantitative plankton work. Aside from the 



