But if the lakes of North America are vast in extent, the navigable rivers are even more gigantic, and afford still wider fields of remunerative employment for steamers.[149] Indeed, until steam-ships were launched on their surface, many of these rivers were altogether unnavigable, and some of them unexplored. Those of my readers who have not visited America, can form only a very imperfect idea of her mighty streams. Some of them, as may be seen by reference to a map of the United States, have their source in the northern parts of the Rocky Mountains, and discharge themselves by the Gulf of St. Lawrence into the Atlantic, while others rising in the west of these mountain ranges flow into the Pacific. Those which have their sources to the east of the Alleghany Mountains find their way by various routes, and through luxuriant valleys, some of them of enormous extent, to numerous outlets on the shores of the Atlantic and on the north-eastern portion of the Gulf of Mexico, while the rivers comprehended under the head of the Mississippi and its tributaries, which spring from that great valley between the Alleghany and the Rocky Mountains, likewise pour their huge volumes of water into the Mexican Gulf, with New Orleans as the chief entrepôt of their now gigantic commerce. The former rivers, upwards of one hundred in number, afford an aggregate amount of more than 3000 miles of ship and boat navigation. But the latter, embracing the parent Mississippi, the Missouri, the Ohio, Arkansas, Red River, and various other tributaries pouring their waters into the giant stream, constitute an aggregate length of no less than 44,000 miles![150] Large steamers now ascend to Pittsburg, a distance of 2000 miles from the Gulf of Mexico. The Missouri, which joins the Mississippi 18 miles above the city of St. Louis and about 1200 miles from the gulf, has an uninterrupted navigation of 2532 miles from its mouth; its tributaries being the Gasconade, navigable for 150 miles; the Osage for 500 miles; the Chariton for 300 miles; the Tansas for 200 miles; and the Yellowstone for 800 miles; while the Moine, which flows into the Mississippi 130 miles above the Missouri, is supposed, with its tributaries, to be navigable for a distance of 1500 miles.