OK TIIK RIVER SAINT LAWRENCE. 



267 



Lachine, is, to a partial extent, reset into floes in the Bay of Laprairie, above the Victoria 

 Bridge, and approaches the St. Mary's current in such masses and quantity as soon to fill 

 up the reaches below, winch, by this time, are more or less blocked up by the accumula- 

 tion of the solid field ice, which commences in Lake St. Peter, and extends for many miles 

 upwards through the low islands and over the shallow rivers between Sorel and Berthier. 

 The next blockade below the city forms at Bouchcrville, where local circumstances favor 

 its collection. The subsequent filling up of the whole surface of the river, for about seven 

 miles of its course, is rarely the work of three days. 



Now the most interesting feature begins to show itself. The ice continues to come 

 down in undiminished quantity and mass, and gradually reduces the channel of St. Mary's 

 Rapid, which results in a rising of the surface to obtain the required discharge through the 

 diminished cross section. Often temporary relief is obtained. The water falls suddenly, 

 and rises again, for like reasons, till at length a uniform " regime" is established. During 

 these sudden rises and falls, the ice coming down with the velocity due to the river, 

 pushes, or shoves, and packs, — rising up and piling itself in mounds on the banks ; 

 covering the wharves — already submerged — to the extent of from two to twelve feet ; 

 mounts, generally, a retaining wall twenty feet above the ordinary level of the river, occa- 

 sionally packing and piling upon it, and obstructing the wide front street, with a mound 

 rising in some instances to the eaves of the warehouses, from twenty-five to thirty-five 

 feet high. 



In other places, as on the low islands in the river, and in shoals in that part of it oppo- 

 site the city, the accumulation goes on, while the projecting upstream side of St. Helen's 

 Island receives its si tare. 



Doubtless the discharge of the river is, to some extent, reduced in quantity through 

 the freezing up of streams whose volume is not regulated by lakes on their respective 

 water-courses; and the discharge of the large lakes is also greatly reduced by the obstruc- 

 tion caused by the barrier of ice across their mouths ; and where, as the head cannot rise 

 in the like proportion (i e. to a degree sufficient to compensate for the reduction of the 

 hydraulic mean depth of the river at its outlet), the mean velocity and discharge must 

 decrease. Such effect, at least, is perceived at the lower parts of the river, where, in 

 general, from six to ten days elapse after the final taking, and all shoving has ceased. 

 The whole surface then falls, except in those parts where the ice has grounded ; for 

 although the ice is, over a good part of the area under consideration, estimated at from 

 ten to twelve feet thick, some portions may be deeper, and actually resting firmly on the 

 bed of the river. There successively appear indications of shoal water ; and we may re- 

 cognize in some of the large mounds on the surface, when it has fallen to its final " regime" 

 of minimum level (which is attained generally in ten days after the final taking), the 



