xlviii FISHES OF ILLINOIS 



formed. Some of these lakes have been drained so thoroughly 

 that they have become small prairies, while in other places they 

 have been unable to cut down their outlets sufficiently. We 

 have, consequently, a series ranging from quiet land-locked 

 ponds with gravel bottoms to marshes differing but little from 

 the ordinary wet prairie or slough, peat bogs, and the dry 

 prairie land. The bed of the swamps is generally more or less 

 peaty, varying in composition from ordinary black swamp 

 muck to true peat. A few of the lakes are from four to seven 

 miles in length and a mile or more in breadth, while the others 

 usually cover only one or two square miles, or even less. These 

 numerous lakelets, ponds, marshes, and bogs furnish, in their 

 aggregate, a considerable storage for flood waters, and the volume 

 of the stream is consequently comparatively uniform and its 

 changes of level are relatively slow. The water of the upper 

 reaches of the river are usually clear except in times of flood, but 

 the lower part of the stream is often very impure. Though 

 much of the river bed below Elgin is in rock, the tributaries often 

 bring large amounts of sediment, and various manufactories along 

 the river discharge a large amount of refuse into the stream, and 

 it has, of late years, become so foul that nearly all fish except 

 carp and other filth-enduring species have been drowned out. 



For a distance of nearly 75 miles from its source Fox River 

 drains only a narrow strip among the morainic ridges of the 

 composite belt, its course being determined by a moraine lying 

 on either side. In this portion of its course its fall amounts to 

 only a few inches to the mile, and its bed expands at frequent 

 intervals into lakes and marshes between which are short 

 stretches having narrow and well-defined channels. The river, 

 here, has no valley, but the stream averages 150 to 200 feet in 

 width, flowing between gravel and clay banks. In some places 

 it runs close to the bluff, while in others a low flood-plain inter- 

 venes. Its tributaries in this section are very small, all occupy- 

 ing deep parallel valleys running in an east and west direction 

 and only turning southward when they reach the lowlands 

 bordering the river. All of the lakes lie along the line of these 

 intermorainic valleys. Among those tributary to Fox River 

 are Lake Geneva, Muskego, and Pewaukee. Fox Lake is 

 simply a widening of the river-bed. 



From the vicinity of Elgin to Yorkville the bed of the river 

 is alternately rock and mud. This is due to the fact that the 

 present course of the river lies almost at right angles to a series 



