This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
In the Glacial epoch a subsidence had begun which continued until it became a very marked feature of the times. The depression was greatest toward the north and especially in the valley of the St. Lawrence; at the mouth of the Hudson, for example, the land stood about 70 feet below its present level, on the coast of Maine 150 to 300 feet, and in the St. Lawrence valley 500 to 600 feet below. The consequence of the depression was that an arm of the sea extended up the St. Lawrence to Lake Ontario, which was little, if at all, above sea-level. Two long and narrow gulfs reached out from this sea, one up the valley of the Ottawa River and the other over Lake Cham-plain, while the Hudson River appears to have been converted into a narrow strait connecting the marine waters of the Champlain basin with those of the Atlantic where New York Bay now is. The lines of raised beaches, the sands and gravels filled with marine shells, and the bones of whales and walruses, are the present evidences of this submergence.
The Champlain subsidence and the reelevation which expelled the sea from the Hudson and Ottawa rivers and from the basins of Lakes Ontario and Champlain also affected the Great Lakes. Lake Iroquois had found a lower outlet than the Mohawk, when the ice withdrew from the Adirondacks, into Lake Champlain, which then discharged into the Hudson, because of the ice barrier to the north. Subsequently this outlet of Lake Iroquois was into the Champlain sea, when the subsidence had drowned the St. Lawrence valley. Just when the Niagara began to flow is not certain, nor when the basin of Lake Erie was refilled, if it were ever emptied, but so long as the upper lakes had their outlet through the Ottawa, the Niagara carried only the discharge of Lake Erie. The elevation which followed the Champlain subsidence was accompanied by a slight tilt of the lake region to the southwest, cutting off the Ottawa outlet and causing the three upper lakes to discharge into Lake Erie. The consequent change in the volume of the Niagara is registered in the sudden increase in the width of its gorge.