550 



from 1.35 at Morris to 3.82 at Chillicothe — an increase of 183 per 

 cent.; while the carbon dioxide drops from 'j.j to 6.1, a loss of 22 

 per cent. — the greater part of both changes occurring, in fact, at the 

 Marseilles dam. It will thus be seen that in judging of the biological 

 effects of pollution and self-purification of the stream, we must take 

 note of the fact that a given increase or decrease in carbon dioxide 

 is accompanied by a loss or gain in oxygen from eight to seventeen 

 times as great. 



SEASONAL PHASES OF CHEMICAL CONDITION 



The combination of our tables and our graphs by seasons, and a 

 comparison of these sets of seasonal data one with another, brings 

 out clearly three phases in river condition which may be called the 

 midsummer, fall, and winter phases, and implies also a fourth or 

 spring phase, for which we have at present no data. The midsum- 

 mer and the winter phases represent, of course, the extremes between 

 which the fall and spring conditions come as intermediate or transi- 

 tion stages. The midsummer phase, with its high temperatures and 

 low stage of water, is characterized by a concentrated pollution and 

 an early and rapid decomposition and deoxygenating process, with 

 lowest oxygen readings at Morris and above the dam at Marseilles, 

 followed by a sudden increase of oxygen below the dam and a 

 gradual rise in ratios thence down the stream to its mouth. The 

 winter phase contrasts with this by a delay of decomposition such 

 that the oxygen ratio is highest at Marseilles, declines slowly to the 

 middle of the river's course, (about at Havana,) and then rises 

 gradually to its mouth. In the autumnal transition phase the oxygen 

 ratio is at its lowest point in the Dresden Heights-^^Iarseilles sec- 

 tion, although much higher there than in midsummer, rises thence 

 slowly to Peoria, and continues on an approximately level line to the 

 mouth of the Illinois. In the spring phase a transition in the oppo- 

 site direction probably gives somewhat similar results, modified, how- 

 ever, by the spring floods, which are usually much larger than those 

 resulting from the fall rains, and by differences in the antecedent 

 seasonal conditions from which this spring transition makes its start. 



These periodical changes in the distril^ution of oxygen and car- 

 bon dioxide within the stream are, of course, consequent upon sea- 

 sonal dift'erences in temperature and stage of water, influenced con- 

 siderably by the upper dam, at Marseilles ; and the above descriptions 

 may perhaps neecl modification to make them applicable to notably 

 unusual vears. 



