Although great natural barriers hindered east-west development in Canada, this circumstance was mitigated by the mighty river and lake systems that provided avenues for the fur trader, missionary, soldier, and settler. Canada's rivers and lakes allowed and, indeed, invited venturesome pioneers to explore the interior of the continent and in spite of natural barriers, to tap its great wealth. The rivers and lakes were essential to the great fur empire; people in canoes brought furs from the farthest extremity of the Canadian Shield to Montreal for exportation to Europe. The first settlements spread along the rivers, since only the rivers provided transportation and communication. Militarily, rivers and lakes were of prime importance; whoever controlled the St. Lawrence and its entrance also controlled Canada.What is the main subject of the passage?