BATHYMETRIC CHANGES ON THE ST-LAWRENCE-RI VER IN THE CONTRECOEUR AREA(QUEBEC) SINCE 1898

Canadian government hydrographic surveys from 1898, 1945 and 1988 are compared to reveal long-term patterns of bathymetric change in the St. Lawrence River, in the Contrecoeur area between Montreal and Sorel. S ince 1850, the hydraulic and sedimentary balance of this reach has bee n affected by repeated dredging to expand the main navigation channel. Using GIS software, detailed bathymetric models of the reach for each of the years 1898, 1945 and 1988 were produced by interpolation throu gh the archival soundings for each of these periods provided by the Ca nadian Hydrographic Service. After careful adjustment of reference wat er planes and planimetric coordinate systems, maps of bed changes were derived by comparing the different models. Major expansions of the na vigation channel can be followed as well as the evolution of the main dredge spoil areas. In particular, considerable volumes of dredge spoi l deposited at the margin of the ship channel between 1898 and 1945 ar e seen to have been re-entrained and dispersed downstream by 1988. Mos t interestingly, the maps of bed changes reveal substantial rates of s ediment deposition over much of the river bed, well away from the ship channel. Over wide areas, deposition of between 0.5 and 1 m of new se diment since 1898 is observed. Bed aggradation has been particularly i ntense in the central channel west of lie Saint-Ours, where it has ave raged 1.5 cm/year between 1945 and 1988. In part at least, this tenden cy to bed aggradation over much of the river may be a reaction to the man-made concentration of the flow in the enlarged ship channel.