In the United States the Great Lakes with the Erie Canal and Hudson River form the most important internal water-way, and by them the continent is penetrated as far west as Duluth, a distance of more than one thousand three hundred miles. The traffic passing out of Lake Superior alone is about one-third greater than that passing out of the Mediterranean Sea at the Suez Canal. Much of this traffic goes across the continent, and the route in question is one of the great commercial highways of the world. The Mississippi River and its branches afford not far from ten thousand miles of navigable waters. Canals connect tributaries of this river with the Great Lakes at Chicago and at several points in Ohio. The development of the navigation of this great water-way was checked by the Civil War, and after the close of the war the great advance in railway building kept its improvement in the background. The general government, nevertheless, has done much to encourage the use of the Mississippi as a commercial highway, and many millions of dollars have been spent in widening and deepening its channel.[8] On the upper river grain and lumber form the chief traffic; on the lower part a large part of the world's cotton-crop starts on its journey to the various markets. On account of the soft-coal fields and the steel manufacture in western Pennsylvania, the commerce of the Ohio River is very heavy, aggregating not far from fifteen million tons yearly. Much of this traffic extends to ports on the Mississippi. The navigable parts of the Hudson and Delaware Rivers are estuaries of the sea or "drowned valleys." In each case navigation extends about to the limits of high tide. Both rivers carry a heavy freight commerce; the Hudson has a passenger traffic of several million fares each year. Nearly every river of the Atlantic coast is navigable to the limit of high tide or a little beyond. Navigation extends to the point where the coast-plain joins the foot-hills. Above this limit, called the "Fall Line," the streams are swift and shallow; below it they are deep and sluggish. As a result, a chain of important river ports extends along the Fall Line from Maine to Florida. River-navigation in Europe in the main is inseparably connected with the great canal systems. As a rule, the lower parts of the rivers are navigable for steamboats of light draught. Some of the smaller streams are made navigable by means of a long steel chain, which is laid along the bed of the stream; the boat engages the chain by means of heavy sprocket wheels driven by steam, and thus wind the boat up and down the river. Ocean steamers penetrate the Amazon Valley to a distance of one thousand miles from its mouth; boats of light draught ascend the main stream and some of its tributaries a thousand miles farther. The Orinoco is navigable within one hundred miles of Bogota. Light-draught boats ascend the tributaries of La Plata River a distance of fifteen hundred miles.