p. 2329C

No.  1008.



(Tidal and Current Survey of Canada.)

General explanation by W. Bell Dawson, M.A., D.Sc., M.E.I.C., superintendent of Tidal and Current Survey, Canada, of results established by investigation of tidal conditions in the Narrows and Hamilton inlet.

The Hamilton River enters a large lake (Lake Melville) which in turn connects with the ocean through the Narrows at Rigolet.  The lake is practically fresh water at the surface, although sea water at the freezing temperature penetrates below this, at the greater depths.
Regarding the Rigolet Narrows and the bay opening eastward from them towards the ocean, the question from the physical standpoint is whether they resemble an ordinary inlet of the sea, or show the characteristics of an estuary or river mouth, as indicated by the tidal conditions.

(I.) In an ordinary estuary, the rise of the tide increases until the tidal undulation reaches the mouth of the river proper, when the rise begins to be 20 cut off and decreased by the river slope.  In the case of Hamilton inlet, our investigations show that in the outer bay with shores which converge from the open ocean towards the Narrows, the tide maintains the same rise from Indian harbour at the mouth of this bay, almost to the Narrows ;  notwithstanding the inflow into the large expanse of Lake Melville.
This indicates a resemblance between the outer bay east of the Narrows, and an ordinary estuary.  The tendency of the tide to increase its range in its progress up the bay, is counteracted by the inflow during its rise, into the immense expanse immediately within the Narrows.  The estuary conditions in this respect thus manifest themselves as far as the configuration permits ;  for the tide is able to maintain its full range to the head of the bay (at Ticoralak island) notwithstanding the adverse influence of the inflow through the Narrows.  In Lake Melville itself, the rise of the tide falls to a small amount, as may naturally be expected in the circumstances.

(II.) It is usual in river mouths for the sea water to find its way along the bottom, especially during the rise of the tide ;  whereas the fresher water of the river itself keeps to the surface.  When the tide begins to rise, there is thus inflow along the bottom while the surface flow is still outward ;  and it takes some time before the surface flow is checked and reversed.  When the

p. 2330

tide is falling, the undercurrent is the first to be checked and reversed, because of the bottom friction and its greater density.
This inflow of an undercurrent of salter water in river mouths whenever the tide begins to rise, has been verified by the investigations of the Tidal Survey in St. John Harbour at the mouth of the St. John river, and in the delta of the Fraser River on the Pacific coast ;  and it appears to be a general feature of river mouths.  This is what occurs here in the Rigolet Narrows.  During the fall of the tide, the conditions are more apt to be modified, as in the case of a river in freshet as contrasted with its summer behaviour ;  or as at Rigolet where there is a large expanse above the Narrows.  The behaviour during the rise of the tide, however, is very typical and characteristic.

(III.)  In both estuaries and tidal rivers, the outward flow or  “ ebb stream ” during the fall of the tide, is stronger and continues for a longer time than the inward flow or  “ flood stream,” which is weaker and of shorter duration because opposed to the current of the river.  As a rule, also, the vertical movement of the tide shows the same inequality between the duration of rise and the duration of fall, as the horizontal movement does.
In the observations obtained by the Tidal Survey at Rigolet, special efforts were made to verify these features ;  and although a steamer was not available for the work, a scow was moored in the middle of the Narrows (where much exposed) to avoid any local disturbance of the main current if nearer shore.  The observations were frequently continued day and night to obtain balanced observations.  A spar-buoy placed near the north side, also afforded more extended observations to supplement those in mid-channel.
The three features of a tidal river were thus made manifest ;  (1) The longer duration of the outward flow, (2) the much greater strength of the current on the ebb than on the flood, and (3) the longer period in the fall of the tide than in the rise.  To make the verification complete, the observations were taken under all the various phases of tidal conditions during the lunar month.

(IV.) The longer duration and greater strength of the flow during the ebb than during the flood, make it clear that the average level of Lake Melville must be at a higher elevation than the average or mean tidal level of the bay to the eastward of the Rigolet Narrows.  The amoutn of the flow is one of the surest indications of difference of leve, even where a direct measurement of the difference may be difficult to determine.
The details of the observations with illustrative diagrams, as well as explanation of their bearing on the questions which here arise, are given in the Report herewith, by Mr. H. W. Jones of this Survey.

Superintendent of
Tidal Surveys.
Ottawa, November 14, 1923.



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