462 
General Comparison of the Illinois River and its Connecting 
Lakes in the Food Resources of a Fishery and in 
Fish Output ; 
Bottom AND LIMNETIC NITROGEN, PLANKTON, ETC. 
In the fact that the Illinois Valley lakes, only in a lesser degree than 
the river itself, are in the spring.or later flood-seasons of all but very ex- 
ceptional years open to receive the sewage-laden water from the upper 
Illinois River and the Chicago Sanitary Canal, they differ materially 
from the isolated glacial type of lake and from ponds or other waters 
which are closed throughout the year to outside sources of nutriment. 
Such supplies as the river lakes receive, they are able to retain in great 
measure when their outflow is reduced to little or nothing by the falling 
of the water levels, but the acquired resources of the river are contin- 
ually drained away by the current, these losses being especially heavy 
when the bottom sediments are being stirred up and scoured out in times 
of flood. Hence the river, as we have seen, is not able to accumulate 
and hold, even in the reach of extremely low-slope between Copperas 
Creek dam and Lagrange, surplus stocks of these substances and the resi- 
dues from their decay as large as the stocks found in the lakes, whether . 
in their deeper open, or in their weedy littoral portions; while in the 
relatively swifter channel below the Lagrange dam the difference is still 
further emphasized. So far as the plankton alone is concerned, the sur- 
plus stocks on hand at a given time per cubic meter of water in Thomp- 
son Lake in 1909—1910 exceeded those in the richest part of the river 
at Havana, opposite, at whatever season, with the percentage of differ- 
ence in favor of the lake largest during the more critical season of low 
productivity, July—November. 
That the central Illinois Valley lakes are also to a considerable ex- 
tent their own furnishers, through the growth and decay of shore vege- 
tation, of their permanent stocks or organic food-materials, is suggested 
by the size of the annual crops or aquatic vegetation, some of which is 
rooted, in their shallower zones; as well as by the fact that the stocks of 
nitrogen, organic carbon and other oxidizable substances in the upper 
layer of bottom soil are appreciably larger in the weedy littoral than in the 
deeper open water. The river, whether in the Havana district or above 
or below, has as offset extremely little weedy shore, where rich stocks 
of similar kind can originate and decay, or where permanent lodgment 
can be furnished for settling suspended organic matters carried in from 
points up stream. 
Bottom AND SHORE FAUNA 
In the case both of the plankton and of the various other food sub- 
stances directly or indirectly usable as food by the*bottom and shore ani- 
mals, our data, as far as they go, point clearly to the presence at all 
