The Champlain Sea was a temporary inlet of the Atlantic Ocean, created by the retreating glaciers during the close of the last ice age. The Sea once included lands in what are now the Canadian provinces of Quebec and Ontario, as well as parts of the American states of New York and Vermont.
The mass of ice from the continental ice sheets had depressed the rock beneath it over millennia, causing it to rebound once the ice melted. This process is gradual and known as isostatic rebound, which can be observed today in the northern Baltic Sea and Hudson Bay. While the rock was still depressed, the Saint Lawrence and Ottawa River valleys, as well as modern Lake Champlain, were below sea level and flooded once the ice no longer prevented the ocean from flowing in to the region.
The sea lasted from about 13,000 years ago to about 10,000 years ago and was continuously shrinking during that time, since the rebounding continent was slowly rising above sea level. At its peak, the sea extended inland as far south as Lake Champlain and somewhat farther west than the site of Ottawa, Ontario. The remaining glaciers fed the sea during that time, making it more brackish than typical seawater. It is estimated that the sea was as much as 150 meters (500 ft) above the level of today's Saint Lawrence and Ottawa Rivers.
The History Channel reported in a show covering the search for Champ, the mythical lake monster, that the Champlain Sea once covered most of New England. This is incorrect, because the area was covered by many glacial lakes, not just one. This is also inconsistent because Cape Cod was also supposedly covered - which would have led to the sea draining into the ocean at many points, even with the land depressed. The modern Connecticut River would have also drained the lake if this was true, leaving tell-tale signs of this.
Modern evidence of the sea can be seen in the form of whale fossils and marine shells that have been found near the cities of Ottawa, Ontario, and Montreal, Quebec, and the existence of ancient shorelines in the former coastal regions. On a clear day atop Mount Pakenham, Ontario, you get a remarkable sense of what the ancient view across the northwest portion of the sea would have looked like. The viewable ancient coastline to the northeast is roughly 40 km (25 mi) away and is known today as the Gatineau Hills in the province of Quebec.