Xxii FISHES OF ILLINOIS 



ably 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 Mississippi near Rock Island. 



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

 ing stream, affording some of the most magnificent water-powers in 

 the country. Dams have been built at numerous places and are ex- 

 tensively used for milling and manufacturing purposes. Although 

 its tributaries, especially at times of freshets, pour their muddy, yel- 

 low streams into its clear waters. Rock River still retains its remark- 

 able clearness almost to its mouth. Here, however, the water is gen- 

 erally quite turbid in consequence of the sewage and other contam- 

 inations which are poured into it. 



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

 consin. The altitude of its source is 875 feet, and of its mouth 536 

 feet, making a total descent of almost 340 feet. The average slope 

 is 1 . 2 feet per mile. Its most rapid section is in Wisconsin, from 

 the mouth of the Catfish to that of the Pecatonica, where for 30 miles 

 the average slope is 1 . 9 feet per mile ; and the next is from Oregon 

 to Sterling and Rock Falls, in which distance of 36 miles the average 

 slope is 1.31 feet per mile. Locally there are more sudden descents 

 than these — as at the Sterling rapids, where there is a fall of 15 feet. 

 The average low-water flow of Rock River is 3,900 cubic feet per sec- 

 ond, and the average yearly flow is 9,944 cubic feet. The average 

 yearly flow is 35 per cent, of the annual precipitation, and the ordi- 

 nary low- water flow is about .36 cubic feet per second per square 

 mile (10th Census). 



There are 10 large lakes tributary to Rock River. These are all 

 in Wisconsin and have a total area of 80 square miles. Among them 

 are Lakes Koshkonong, Mendota, Monona, and Beaver Dam. Lake 

 Koshkonong, in southwestern Jefferson county, is an expansion of 

 Rock River 2 miles wide and 10 miles long, with its foot 6 miles 

 above the mouth of the Catfish River. A large dam has been erected 

 across its outlet and is controlled in the interests of the water-power 

 below. To this and the dams of several other smaller tributary 



