146 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



bring into it a heavy deposit of glacial gravel. The easternmost gap is 

 along Negley Run, immediately north of East Liberty, and has a width of 

 nearly one-half mile. The middle gap is along Haights Run, less than 

 a mile west of Negley Run. This gap is scarcely more than one-fourth 

 mile in width. The third or westernmost gap sets in at Allegheny cemetery 

 and extends westward to the base of Herron Hill, being nearly a mile in 

 width. These gaps were filled by glacial gravel to a height of about 

 75 feet above the rock floor of the old oxbow of the Monongahela, their 

 highest points being 970 to 975 feet above tide, as determined by Jillson 

 with Locke level. The gravel extends but little into the old channel of 

 the Monongahela, a feature which seems to indicate that the gravel-bearing 

 water from the Allegheny there encountered a lagoon with but little 

 current. 



All the important tributaries of the Lower Allegheny enter from the 

 east, the divide on the west being but 10 to 20 miles distant from the river. 

 The largest eastern tributary, Conemaugh River, has a drainage area of 

 about 1,800 square miles, or nearly one-sixth of the entire basin of the 

 Allegheny and fully one-third of the Lower Allegheny Basin. The Clarion 

 has a drainage area of about 1,200 square miles, Redbank River 550 square 

 miles, and Mahoning Creek 400 square miles. These four streams drain 

 nearly four-fifths of the area tributary to the Lower Allegheny. 



Rising, as these eastern tributaries do, on the border of the Allegheny 

 Mountains, they have very rapid fall and are subject to great freshets in the 

 spring, at which time the melting of snow and heavy rains often unite to 

 swell their volume. The Johnstown flood, on the Conemaugh, is a con- 

 spicuous instance of the disasters occasioned by siich freshets. These 

 streams are also subject to extremely low stages in the summer months. 

 Porter estimates that the Conemaugh is at times reduced to less than 90 

 cubic feet per second, or scarcely two-fifths its proportion of the average 

 low- water discharge of tlie Allegheny River (1,330 cubic feet per second).^ 

 It is conspicuously lower than the low-water discharge of tributaries that 

 drain drift-covered districts. Thus, French Creek, which has a drainage 

 area of but 1,130 square miles, is estimated to have a low- water discharge 

 of about 700 cubic feet per second.^ 



1 Water power of the Ohio River Basin, etc., by Dwight Porter: Tenth Census of United States, 

 1880, Vol. XVII, Pt, II, pp. 442,445. 



2 Op. cit., p. 448. 



