366 



the prevailing direction of the winds and the current together seem to 

 keep this soiHng material largely confined between the channel and 

 the bank as far as the foot of the lake, where the first extensive mix- 

 ing of lake water and local sewage appears ordinarily to take place. 

 The general orientation of both the channel and the adjacent wide- 

 waters is such, however, that in the fall of the year and occasionally 

 during the summer season, north or slightly north of west winds may 

 cause much more mixing of the sewage with the lake water eastward. 



Soft mud bottom was met at all stations in the lower lake proper. 

 Harder mud and shell bottom was found in midstream at the Narrows, 

 and nearly clean sand at the P. P. U. Bridge, where the current is 

 four to five times that in the channel opposite Workhouse Point and 

 six to seven times midstream current riieasured near the middle of 

 the middle lake in the summer of 1930. 



Since the first cross-section in the lake proper was above most of 

 the main sewers, and we confined our collecting below that place to 

 the channel and the open lake eastward of it, no especial foul odors 

 were recorded at any of the lower lake stations in the 1922 summer 

 season. Bubbling was observed in abundance at most stations op- 

 posite Workhouse Point, as also at the farthest eastward stations op- 

 posite Fulton Street, but in the middle of the main channel and at 

 most open lake stations opposite Fulton Street neither bubbles nor 

 unusually bad odors were noted. 



Little restoration of the killed-out vegetation in the southeastern 

 part of the lower lake had occurred in the two years since 1920, the 

 thin Potamogeton patcli on the east side facing Liberty Street looking 

 about as it did in the summer of 1920. Traces of Vallisneria were 

 brought up by the dredge at the station nearest the we.^t shore off 

 Workhouse Point, but no vegetation was encountered in any other 

 hauls. 



Dissolved O.xvgen 



Both in the summer of 1922 and of 1920 the supply of dissolved 

 oxygen a foot from the bottom in mid-channel at the foot of Peoria 

 Narrows ranged between 50 and 75 per cent of saturation, numerous 

 samples taken between the middle of July and the middle of Sep- 

 tember 1920 averaging 5.1 to 6.4 parts per million, and eight samples 

 taken in September 1922 averaging 4.8, with a range from 3.6 to 6.2 

 p. p, m.' Soon after the middle of July 1920 almost as high or higher 



