HYDROLOGY OF NEW YORK 



365 



Geologically, Lake Ohamplain lies mostly in the horizon of the 

 Trenton limestone, the drainage being from the Laurentian 

 granites and Plutonic norites. 



Discharge measurements of Hudson river at Mechanicville. 

 Measurements of the flow of the Hudson river at Mechanicville 

 have been made over the dam of the Duncan Company. In 1887 this 

 company began daily measurement of the amount of water flow- 

 ing in the Hudson river at their mill. 1 With the exception of one 

 or two days, this record has been kept for every working day 

 since October 1, 1887. A record has also been kept of the num- 

 ber, size, and kind of turbine water wheels in use for the same 

 period. The Duncan Company placed all this material at the 

 disposal of the survey of the upper Hudson valley, thus enabling 

 one to compute the mean daily flow of the river for each working 

 day from October 1, 1887, to November, 1897. The flow of Sun- 

 days and holidays, when no observations were taken, has been 

 assumed as a mean between the preceding Saturday and the fol- 

 lowing Monday, etc. The* dam is a substantial structure of 

 masonry 16 feet high, with a length of 794 feet between the abut- 

 ments. The crest is stated to be perfectly level, and from all 

 that can be learned it appears that the daily observations have 

 been taken with such care as to leave no reason for doubting 

 that this is a fairly accurate exhibit of the daily flow of the 

 stream for the period covered. 



The catchment area of the Hudson river above the Mechanicville 

 dam is taken at 4500 square miles, although a recomputation 

 from the latest maps made for the Board of Engineers on Deep 

 Waterways gave 4507 square miles. This is only 0.15 of one per 

 cent different from the former computation of 4500 square miles 

 and is not enough to make it worth while to recompute the 

 runoff. 



The flow of the Hudson river at Mechanicville prior to 1899 has 

 been computed by using the East Indian engineers' formula for 

 flow over a dam, and when flashboards are on, the Francis formula 

 for a sharp-crested weir has been used. Since that time, the 

 computations have been made by R. 1*. Bloss, Engineer of the 



l Ann. Rept. of State Engineer and Surveyor of New York. 1895, p. 104. 



