Inland Waters.

Rivers. The rivers of North America empty their waters into the oceans that wash its shores. There are areas on the plateaus between the Cordillera ranges whose waters do not reach the oceans, but these are very small in comparison with the undrained areas of Asia and Africa.
The Cordilleras are the dividing line between the Pacific and At­lantic drainage-basins. The Atlantic basin is, therefore, much larger than the Pacific.
The principal river of the Atlantic Basin is the Mississippi, North America's greatest river (from the Indian = big river). It rises in the interior of the continent and, after a series of bends, flows south across a plain into the Gulf of Mexico. The Mississippi is joined on both sides by tributaries, those on the right hand coming from the Cordilleras, those on the left from the Appalachians. Its longest tributary is the Missouri, which is longer even than the Missi­ssippi itself. If the distance between the source of the Missouri to the mouth of the Mississippi be considered its length, then the Missouri-Mississippi is the longest river on earth (6,800 kilometres). It carries, however, a far smaller volume of water than the equatorial rivers: the Congo of Central Africa and the Amazon of South America, for instance. The level of the Mississippi is very unstable. Its volume is greatest in spring, owing to snows melting on the plains. Summer rains, moreover, cause huge floods over fields, villages and even whole towns. The Mississippi deposits much silt on its bed, forming shallows and islands that hinder navigation. It forms a large delta at its estuary. The Mississippi Delta is a whole network of channels amidst masses of reeds and riverside marshes. Every year it is pushed another hundred metres out into the Gulf of Mexico.
A number of great rivers fall into the Arctic Ocean. They are fro­zen for the greater part of the year and flow through almost unin­habited regions.
The rivers flowing from the mountains into the Pacific are mostly short, rapid and unstable in depth. One of the most important rivers

Fig. 155. The Grand Canyon of the Colorado. The deep gorge is edged by craggy walls. At the bottom flows a river.

of the Pacific Basin is the Colorado with its source in the snow-clad mountains. It flows across a plateau, carving a deep gorge edged by craggy walls which is called the Grand Canyon (Fig. 155) and finally falls into the Gulf of California.
The larger rivers of North America could be widely used as cheap means of communication and important sources of energy. They have never received much development, however, because of railway and automobile competition.
There are large hydroelectric power-stations on the Colorado and on a few other North American rivers, but they are smaller than those in the Soviet Union.
Lakes. There are many lakes in North America. As in Europe, they are situated in the northern part of the continent. Five lakes stand out especially. These are Lake Superior, Lake
Michigan, Lake Huron, Lake Erie and Lake Ontario. Together they are called the Great Lakes of North America. Any one of these lakes is larger than Lake Ladoga, the largest lake in Europe. Lake Superior is the largest freshwater lake in the world (over 80,000 square kilometres). The lakes lie at different altitudes and are connected by short, swift channels (Fig. 156).
Lake Erie and Lake Ontario are connected by the Niagara River (one kilometre wide). Throwing its waters over a ledge 50 metres high, it forms the great Niagara Falls (Fig. 157). The magnificent combined effect of falling water, dark-green waves, white foam and sparkling spray provides a great attraction to tour­ists. The falls furnish the power for a number of hydroelectric power-stations.
The Great Lakes have an outlet to the ocean through the rapid and deep St. Lawrence River.

They are important inland water-ways and widely used for trans­port. A number of canals have been built: one to avoid the Niagara

Falls, another to permit ships to move between the Great Lakes and the Atlantic Ocean, still another to connect the lakes with the Mis­sissippi. Navigation along the St. Lawrence has been improved.
On the plateaus between the Cordillera ranges there are a number of salt lakes without outlets. During the thaw in spring they are filled with water. In the summer they become smaller and are edged with a broad border of dry silt mixed with salt.
Questions and Assignments.

• On the outline map trace the dividing lines between the Pa­cific, Atlantic and Arctic drainage basins. Name the rivers and lakes of North America.
• Compare Lake Superior and Lake Baikal. In what respect is the former superior, in what inferior?