When the Breton sailors, steering their fishing smacks almost in the wake of Cabot, began to fish in the St. Lawrence gulf, and to traffic with the natives of the mainland for peltries, the problem of how the interior of North America was to be explored was solved. The water-system composed of the St. Lawrence and the Great Lakes is the key to the continent. The early explorations in a wilderness must be by water-courses--they are nature's highways. The St. Lawrence leads to the Great Lakes; the headwaters of the tributaries of these lakes lie so near the headwaters of the rivers that join the Mississippi that canoes can be portaged from the one to the other. The Mississippi affords passage to the Gulf of Mexico; or by the Missouri to the passes of the Rocky Mountains, where rise the headwaters of the Columbia, which brings the voyageur to the Pacific. But if the explorer follows Lake Superior to the present boundary line between Minnesota and Canada, and takes the chain of lakes and rivers extending from Pigeon river to Rainy lake and Lake of the Woods, he will be led to the Winnipeg river and to the lake of the same name. From this, by streams and portages, he may reach Hudson bay; or he may go by way of Elk river and Lake Athabasca to Slave river and Slave lake, which will take him to Mackenzie river and to the Arctic sea. But Lake Winnipeg also receives the waters of the Saskatchewan river, from which one may pass to the highlands near the Pacific where rise the northern branches of the Columbia. And from the lakes of Canada there are still other routes to the Oregon country.[61] At a later day these two routes to the Columbia became an important factor in bringing British and Americans into conflict over that territory. In these water-systems Wisconsin was the link that joined the Great Lakes and the Mississippi; and along her northern shore the first explorers passed to the Pigeon river, or, as it was called later, the Grand Portage route, along the boundary line between Minnesota and Canada into the heart of Canada. It was possible to reach the Mississippi from the Great Lakes by the following principal routes:[62] 1. By the Miami (Maumee) river from the west end of Lake Erie to the Wabash, thence to the Ohio and the Mississippi. 2. By the St. Joseph's river to the Wabash, thence to the Ohio. 3. By the St. Joseph's river to the Kankakee, and thence to the Illinois and the Mississippi. 4. By the Chicago river to the Illinois. 5. By Green bay, Fox river, and the Wisconsin river. 6. By the Bois Brule river to the St. Croix river. Of these routes, the first two were not at first available, owing to the hostility of the Iroquois.