8o Types of Aquatic Environment 



It flows in thin sheets over broad ledges of dark colored 

 rocks that are exposed to the sun, and it falls over pro- 

 jecting ledges in broad thin curtains, outspread in con- 

 tact with the air. 



The curves for the two winter months, show less 

 concurrence, and it is strikingly apparent that during 

 that period when the creek was ice-bound (Dec. 15- 

 Jan. 31) the water temperature showed no relation to 

 air temperature, but remained constantly at or very 

 close to o°C. (32° F.). 



Forbes and Richardson (13) have shown how great 

 may be the aerating effect of a single waterfall in such 

 a sewage polluted stream as the upper Illinois River. 

 ''The fall over the Marseilles dam (710 feet long and 10 

 feet high) in the hot weather and low water period of 

 July and August, 191 1, has the effect to increase the 

 dissolved oxygen more than four and a half times, rais- 

 ing 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 

 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 * * * The beneficial effect is 

 greatest when it is most needed — ^when the pollution is 

 most concentrated and when decomposition processes 

 are most active." 



Ice — The physical conditions that in temperate 

 regions have most to do with the well- or ill-being of 

 organisms living in running water are those resulting 

 from the freezing. The hardships of winter may be 

 very severe, especially in shallow streams. One may 

 stand beside Fall Creek in early winter when the thin 

 ice cakes heaped with snow are first cast forth on the 

 stream, and see through the limpid water an abundant 



