LONGITUDINAL STUDIES 109 



The obstruction of the hard layer encountered always produces local 

 swift water. Above this the water may be sluggish and the area reduced 

 to the general level of the obstruction. In the case of rejuvenation the 

 head of erosion proceeds upstream ; the part of the stream above the point 

 to which erosion has reached is sluggish and is sometimes called the pre- 

 erosion stream. 



Of the rivers and creeks which we have considered, nearly all the 

 larger ones are sluggish or pre-erosion in their upper courses. This is 

 true of the DesPlaines, which is held in this condition largely by rock 

 at Riverside. Hickory Creek (Fig. 66) is also of this type, the head of 

 erosion being at Marley. In passing from source to mouth of such a 

 stream we find formations arranged as follows: In the upper sluggish 

 courses of all the streams mentioned we find (i) sluggish creek or 

 river formations, (2) chiefly swift- water formations below the sluggish, 

 (3) chiefly gravel bottom formations below the swift-water formation, 



B 



FIG. 66. Diagrammatic profile of Hickory Creek: A, source; B, mouth; C, head 

 of erosion; D, rock outcrop. The figures below refer to the columns in Table XXI 

 and represent parts from which fish were collected. 



and (4) typical sluggish river formations farthest downstream where 

 the vegetation, silt, and sand formations are arranged much as in 

 the Fox River. 



Tables XVIII, XXI, and XXII and Figs. 67-69 show the longi- 

 tudinal distribution of fishes in six streams. A few moments' study and 

 comparison of these tables will make the following facts evident: 



a) The only species in the youngest stream of the North Shore 

 series is at the headwaters of all the others. 



6) The species found in County Line Creek are found in the same 

 order in the upper courses of Pettibone Creek and Bull Creek; additional 

 species are found farther downstream in the larger streams. 



c] The same species are at the headwaters of Thorn-Butterfield and 

 Hickory creeks and in the upper courses of the North Shore streams. 

 Other species are with them. The species of the North Shore streams 

 are crowded together in these large streams which have permanent 



