10 ILLINOIS BIOLOGICAL MONOGRAPHS [10 



lake, and from Golden lake on the west 12 miles east to Pewaukee lake and 

 the Fox river. Within the rough rectangle thus formed lie no fewer than 

 forty-five lakes of various sizes, ranging from Pewaukee lake with an area 

 of 2,297.8 acres down to such small bodies of water as Washtub and Mud 

 lakes, with an area of three or four acres only. Of these, twenty-five of the 

 largest and most important have been surveyed, and their depth, area, 

 shore-line and general topography recorded by Birge and Juday (1914). 

 These lakes show a total water surface area of 9,971 acres. Besides these 

 lakes five river systems drain the region, and there are many streams of 

 lesser magnitude. 



Outside the Oconomowoc-Waukesha group the county contains a 

 number of lakes of considerable size, scattered over the remaining three- 

 quarters of the area. The largest of these, Muskego, near the town of that 

 name in the southeast corner of the county, nearly equals Pewaukee in 

 size, but it has never been surveyed, and its exact area is unknown. Here, 

 too, lie Little Muskego, Denoon, Muckwonago, Phantom, Eagle, and 

 many smaller unsurveyed lakes, all of which, however, are included in this 

 report. In the northeast corner of the county there are no lakes, but here 

 is found the Menomonee river, which introduces the Lake Michigan 

 drainage area into the scope of the investigation, along with which come 

 several species of fish found in no other part of the county. The lakes and 

 river systems will be listed subsequently. 



The region under discussion is of importance in connection with the 

 work of Dr. Stephen A. Forbes and R. E. Richardson, together with their 

 associates in the Illinois State Natural History Survey, because within it 

 lies the source of the Fox river of Illinois, together with a great number of 

 lakes and several river systems which flow into the Rock River and the 

 Illinois, on which rivers the Illinois Natural History Survey is working so 

 intensively. The region is, therefore, one of the great tributary areas to the 

 river systems upon which they are focusing their energies. 



The lakes of Waukesha county owe their origin to the action of the 

 great ice sheet which featured the pleistocene epoch of the quaternary 

 period. There were several great movements north and south of the 

 Labrador glacier and other huge ice masses hundreds of feet thick. These 

 glaciers crawled southward from their northern source, then slowly re- 

 treated, causing great changes both in the topographic and climatic condi- 

 tions of the regions invested, and it is largely to the influence of this glacial 

 movement that is due the present day distribution of the plants and 

 animals both within the territory concerned and the adjacent regions. 

 There seem to have been at least three — probably five — more or less dis- 

 tinct advances of the ice, the three most important being the Pre-Wiscon- 

 sin, the Early Wisconsin and the Late Wisconsin, and it is to the last of 

 these invasions that is traced the origin of the Waukesha county lakes. 



