549 



Mississippi River. The data thus obtained resemble each other closely 

 in the general trend of the series of determinations, from a very low 

 ratio at Lockport, with a considerable and rather steady rise to Peo- 

 ria (10.5 parts per million), and nearly uniform ratios thence to 

 the mouth of the Illinois. The later series, of November 12 to 19, 

 runs on an average considerably higher than the earlier, November 

 I to 7, but notwithstanding the saturation percentage of 87 at the 

 mouth of the river (10.7 parts per million) it presents the strongest 

 contrast with the Mississippi River, the midstream waters of which 

 contained at this time 14.2 parts per million, amounting to 114 per 

 cent, of saturation. There was no place on the Illinois on either of 

 these trips where the oxygen ratios were less than 4.6 parts per 

 million. 



THE EFFECT OF A DAM ON DISSOLVED GASES 



The fall over the Marseilles dam in the hot weather and low-water 

 period of July and August, 191 1, had the effect to increase the dis- 

 solved oxygen more than four and a half times, raising it from an 

 average of .64 parts per million to 2.94 parts. On the other hand, 

 with the cold weather, high oxygen ratios, and higher water levels 

 of February and March, 19 12, and the consequent reduced fall of a 

 larger volume of water at Marseilles, the oxygen increase was only 

 18 per cent. — from 7.35 parts per million above the dam to 8.65 

 parts below; and in August and September, 1912, the weather being 

 still cooler and the water lower than in the midsummer of the pre- 

 vious year, the increase was only 'jy per cent. — from 2.05 parts per 

 million above the dam to 3.62 parts below. It should be noted, how- 

 ever, that this beneficial effect is greatest when it is most needed — 

 when the pollution is most concentrated and decomposition processes 

 are most active. In the absence of a dam at this point, the recovery 

 of oxygen used up in decomposition would be greatly retarded in 

 midsummer, and heavily polluted water would be carried much 

 farther down the stream. 



A reverse but much less pronounced effect is produced at the dam 

 upon the carbon dioxide content of the waters of the stream. This 

 was diminished, in the summer of 191 1, from 8.2 parts per million 

 above to 6.48 parts below the dam — a reduction of 21 per cent. The 

 ratio of loss of carbon dioxide was thus only a seventeenth part of 

 the ratio of gain in oxygen. A similar statement may be made con- 

 cerning losses and gains of these gases for the upper Illinois River 

 as a whole. Taking the means of the oxygen ratios at each point for 

 the period from July 28 to August 29, 191 1, we find them rising 



