THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS XXV 



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 Peca- 

 tonica, 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 second, and 

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

 flow is 35 per cent, of the annual precipitation, and the ordinary 

 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 lakes is very 

 largely due the maintenance of a comparatively uniform flow 

 in dry and severely cold seasons. 



In Wisconsin the banks of Rock River are quite low and 

 rolling, but at Janesville the river enters a wide preglacial 

 valley which it follows to a point a little below Rockford, 111. 

 The stream then turns abruptly westward while the valley 

 continues southward toward the Illinois River, the valley of 

 which it enters at Hennepin. This changing of the river course 

 is doubtless due to the Wisconsin moraine which was left across 

 its path when the glacier retreated. The water then found an 

 easier outlet through the preglacial channels of some of its 

 former tributaries. The valley averages about 3 miles in width, 

 although in places it reaches a width of 5 miles. Most of the 

 way the river follows the western edge of the valley, although 

 just above Rockford it crosses to the eastern side and then back 

 again. Thus, the eastern banks are usually low while those on 

 the west are high and steep, in some places rising 75 feet above 

 the water. W^hen the stream turns westward below Rockford, 

 it runs for 50 miles through a narrow valley to a point a few 

 miles below Dixon. For a large part of this distance it flows 

 through the preglacial channels spoken of above. Through this 



