XXIV FISHES OF ILLINOIS 



swamps and numerous small lakes, the drainage being almost 

 entirely independent of preglacial lines and consequently im- 

 perfectly developed. The overflow from the swamps is gathered 

 into little meandering streams which have cut only small chan- 

 nels in the soil. The rest of the basin is older country undulat- 

 ing, well-drained, and forming excellent farm-land except along 

 Green River, where there are many swamps and sand-hills. 



ROCK RIVER 



Rock River is, of course, the principal river in the system. 

 It rises in Dodge county, Wisconsin, in what was formerly Lake 

 Horicon, but is now drained and has become an extensive marsh. 

 The lake, which existed until 1868, although a body of water 

 formed by an artificial dam, yet occupied the site of an ancient 

 lake caused by the body of drift which formed a natural barrier 

 to the passage of the water. Gradually this was eroded and the 

 lake drained, probably through the same passage which now 

 forms the channel of Rock River past the village of Horicon. 

 A dam 200 feet in length, erected at this point, would raise the 

 water 10 feet and restore the old lake to a large extent (10th 

 Census). Leaving Horicon, the river runs through the eastern 

 part of its basin until opposite Oconomowoc, where it turns 

 abruptly northwest to Watertown. Here it suddenly bends 

 again to the southwest, following this direction until it reaches 

 the Illinois state-line near the center of its basin. From here it 

 winds and curves toward the southwest, following at first the 

 center of the basin, but finally running decidedly nearer its 

 western boundary line. It empties its waters into the Missis- 

 sippi near Rock Island., 



Throughout its course Rock River is a bright, clear, swiftly 

 flowing stream, affording some of the most magnificent water- 

 powers in the country. Dams have been built at numerous 

 places and are extensively used for milling and manufacturing 

 purposes. Although its tributaries, especially at times of fresh- 

 ets, pour their muddy, yellow sediments into its clear waters, 

 Rock River still retains its remarkable clearness almost to its 

 mouth. Here, however, the water is generally quite turbid in 

 consequence of the sewage and other contaminations which are 

 poured into it. 



The river is nearly 300 miles long almost one half lying in 

 Wisconsin. The altitude of its source is 875 feet, and of its 

 mouth 536 feet, making a total descent of almost 340 feet. The 



