OF THE RIVEK SAINT LAWRENCE. 



279 



(the depth being about forty-five feet), through about half a mile of its course. The river, 

 also, for several miles below, has about double the width mentioned, and is from twenty- 

 five to thirty-five feet deep, with a fall of about six inches per mile, producing a surface 

 velocity of 1.45 miles per hour. The flatter surface last mentioned is generally covered 

 with ice throughout the winter, but the rapid at the lake outlet is rarely covered more 

 than once in five years. And under these circumstances, it will be observed that the 

 hydraulic mean depth will be reduced from forty to forty-five per cent., by addition of the 

 coating of irregular masses of ice forming the surface, which thus adds to the wetted 

 perimeter, our divisor in hydraulic calculations. 



The same facts have been observed to take place at the discharge of Lake Erie, near 

 Buffalo, which is described by Major Lachlan, in the Canadian Journal, of 1854, at the 

 breaking up of the ice of that year, as having had the effect of reducing, for forty hours, 

 the discharge of the; Niagara River, so as (according to other testimony), to have reduced 

 the apparent discharge of the cataract by at least one-half, and on which occasion opera- 

 tions were carried on by the mill-owners, on the American side of the river, far out into 

 the stream. The writer also observed, in September, 1857, a rise of two feet nine inches 

 in the level of the water at the Ferry wharf, below the Falls, which took place in the 

 course of one night. This result was not due to rain, nor to any other circumstance, but 

 the continuance, for about twelve hours, of a heavy gale from the southwest, which had 

 the effect of raising the head, and thus increasing the discharge through the rapids at 

 Buffalo, so as to require the additional head of two feet nine inches, in the reach of the 

 river, immediately below the Falls, to enable that deep section of the river between the 

 Falls and the Suspension Bridge, to carry off the increased volume. 



The writer by no means desires to imply that all these fluctuations in levels arc to be 

 explained by the effects of the accumulation of ice, or by its entire absence ; as it is 

 obvious that these only form one of the many circumstances which regulate the very in- 

 teresting phenomena to which reference has been made. 



It will be observed from the facts above stated, that notwithstanding the extensive area 

 of these great lakes, which act in general as compensatory reservoirs, in equalizing the 

 discharge to an almost uniform quantity ; yet there arc times, as before explained, when 

 an excessive discharge, as well as the reverse action, will introduce abnormal conditions 

 which would have to be eliminated in any calculations of actual quantity ; and it may be 

 stated, as an interesting fact bearing on this subject, that tire rise of water, to the extent 

 of one foot, on Lake Huron, for about twenty hours, in the summer of 1858, appears to 

 have affected the discharge; of the whole of the River St. Clair, the Detroit River, and the 

 intermediate Lake St. Clair, throughout its length, the increase twenty miles down having 

 been about seven inches, and at Detroit two and three-quarter inches: the central surface 



