126 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



that vicinit}^ exceed 2,500 feet. In the first 20 miles, to Coudersport, Pa., 

 the fall is very rapid, but below this point the valley has been silted up 

 and the rate of fall becomes very much reduced. In the 30 miles from 

 Port Allegany, Pa., to Olean, N. Y., the fall is only about 50 feet, or less 

 than 2 feet per mile, and this low rate continues to Salamanca, 20 miles 

 below Olean. The following condensed statement of the fall of the river 

 from Olean to Pittsburg is taken from Roberts's Report on the Survey of the 

 Allegheny River.^ 



Fall of Allegheny Biver from Olean, N. Y.^to Pittsburg, Pa. 



per mile. 



First 20 miles below Olean ^ 1-7 



Second 20 miles - 3-7 



Third 20 miles - - 5.0 



Fourth 20 miles - - - 3. 5 



Fifth 20 miles 3.5 



Thirty-two miles to Franklin - 3. 



Franklin to Pittsburg, 123 miles - 2. 1 



Average Olean to Pittsburg, 255 miles 2. 8 



Average Olean to Franklin, 132 miles - 3.4 



From the same report it appears that there are 190 riffles between 

 Olean and Franklin, with an average length of 617 feet and an average 

 descent of 1.6 feet. These riffles have a combined length of 22.2 miles and 

 an aggregate descent of 304.77 feet. This leaves 141.49 feet descent for 

 the 110 miles not occupied by riffles, or a descent of less than 1.3 feet per 

 mile. From an earlier report by Roberts " it appears that between Frank- 

 lin and the mouth of Conemaugh River, 30 miles above Pittsburg, there 

 are 58 riffles, with an average length of 1,460 feet and an average descent 

 of 2.46 feet. There are 11 riffles in the lower 30 miles of the river, but 

 their length and descent have not been ascertained. 



The most formidable riffle in the entire length of the portion surveyed is 

 "McGinnis Rapids," about 8 miles above the mouth of the Clarion River, 

 where a descent of 11.23 feet is made in a distance of 6,900 feet. This is 

 described to be a connected series of rapids, shoalest at the head. About 

 one-half mile above the head of the rapids the river has a rock bed, but 

 it is not certain that the rapids are over rock ledges; nor is it certain th9,t 

 the rock ledge over which the river flows extends entirely across the valley 

 at a level as high as tlie river bed. 



' Senate Doc. No. 89, Forty-sixth Congress, second session, 25 pages. 

 ^ House Doc. No. 21, Forty-fifth Congress, third session, 17 pages. 



