HYDROLOGY OP NEW YORK 



481 



was rather low August 23. Hence, Mohawk flats were iu shape 

 to absorb a considerable portion of the water delivered upon them 

 into the interstices below the surface. Assuming, as already 

 stated, 30 square miles to be more or less effected by such an in- 

 flow, and further assuming the interstices of the soil of this 30 

 square miles to be for the first 8 or 10 feet of depth about 30 per 

 cent to 40 per cent of the whole volume, we reach the conclusion 

 that the temporary storage of the flats may have been as much 

 as 3,000,000,000 to 4,000,000,000 cubic feet, which sufficiently ex- 

 plains why the outgo at Little Falls did not exceed 12,000 to 

 14,000 cubic feet per second. 



The foregoing figures do not take into account the extreme maxi- 

 mums which may be expected on this stream. So far as can be 

 now seen, from 40,000 to 50,000 cubic feet per second delivery 

 into the flats above Little Falls is rather common. The extreme 

 maximum must be considerably greater than this. Without 

 being able to cite specific floods proving the statement, the 

 writer is nevertheless disposed to say, taking into account the 

 known maximum floods of other streams of the State — Cheniung, 

 Genesee and Croton rivers — that at some time the Mohawk river 

 above Little Falls may yield something like 60,000 to 70,000 cubic 

 feet per second, or at the rate of perhaps 46 to 54 cubic feet per 

 second per square mile. 



A study of the flood-flows of the Mohawk river would be incom- 

 plete without some reference to the surface geology of the catch- 

 ment area. The main trough of the valley lies entirely within the 

 horizon of the Hudson river and Utica shales, where the soils, aside 

 from immediately in the flat, level valley, are heavy and compact. 

 On the north side, beyond the horizon of the shales, Trenton lime- 

 stone and calciferous and sand-rock appear. At Little Falls, St 

 J ohnsville and Amsterdam these formations extend down into the 

 valley to the thread of the stream. To the north of the Trenton 

 limestone and calciferous sand-rock the Laurentian granite ap- 

 pears, the headwaters of West and East Canada creeks lying in this 

 formation, while tl^e less extended streams on the north side ter- 

 minate either in the shale or limestone and sand-rock horizons. 

 In the vicinity of Fonda, Johnstown, Gloversville and Mayfield 

 the shale formations extend for some distance to the north of the 

 trough of the valley, as also they do farther east, to the north of 

 Schenectady. 



