17] STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 17 



so common to the lakes and streams of Illinois. The clearest of all the lakes 

 is Upper Nashotah, with Oconomowoc second and Lower Nashotah third. 

 By use of the Secchi disc, a white enamel disc 10 cms. in diameter, the 

 visibility of these lakes has been tested, with the following results: 



The shoreline of all of the lakes is constantly undergoing change and, 

 although this alteration is almost imperceptible from year to year, yet 

 it has an important effect upon the lakes. This action is two-fold: 1) that 

 of the waves, and 2) that of the ice. The constant attack of the water upon 

 the shores is constantly eating away the shoreline and the material thus 

 removed is carried out to be deposited eventually in the lake. This action 

 has in the course of time profoundly changed the shoreline of the lakes, and 

 has built up a characteristic series of sand bars which extend around the 

 lakes at a varying distance from the shore. This is particularly true in the 

 case of all lakes lying in a sand-gravel formation and less true in the case 

 of those small lakes lying in a loose, black muck soil. This muck tends to a 

 far more uniform deposition and consequently is deposited more nearly 

 over the entire lake bottom, and bars do not occur. Such facts are of 

 extreme importance to the fish life of the lakes as the presence or absence 

 of bars may in many cases mean the presence or absence of breeding 

 grounds, which in turn limits the possible species population. 



As has been stated, most of the larger lakes of the county have been 

 surveyed by the Wisconsin Geological and Natural History Survey, and 

 their hydrography and morphometry is well known. The results have been 

 published by Birge and Juday (1914) and the reader is referred to this 

 work if details are desired. The following table has been compiled from 

 that source in order to bring the general facts into this paper for ready 

 reference. 



