A divide is the elevated boundary separating areas that are drained by different river systems. For this reason, the feature is often called a drainage divide. Water flowing on one side of a divide empties into one body of water, while water flowing on the other side empties into another. Divides range in height from a slight rise in the land to the crest of a mountain range. A divide on the low-lying flood plains of northern Belgium, for example, may rise no higher than a few meters. At the other extreme, the Andes—one of the longest mountain ranges in the world—form a divide in South America. A mountain range that forms a divide is called a dividing range. There are three major types of divides: continental divides, major divides, and minor divides. Continental Divides A divide that separates different watersheds of a continent is called a continental divide. Because a continent may have more than two watersheds, a continent may have more than one continental divide. North America, for instance, has about five continental divides. These continental divides separate the drainage systems leading to the Pacific Ocean, Arctic Ocean, Atlantic Ocean, Hudson Bay, and Gulf of Mexico. The most familiar continental divide in North America separates the watersheds of the Pacific Ocean in the west and the Arctic Ocean, Hudson Bay, Atlantic Ocean, and the Gulf of Mexico in the east. The so-called Great Divide roughly follows the crest of the Rocky Mountains. Major Divides A major divide forms when two rivers flow into the same body of water but do not meet. A major divide can be large or small. The watersheds of China’s Yellow and Yangtze Rivers form a major divide, for example. The Yellow River forms a watershed in China’s north. It empties into the Bohai Sea, part of the Pacific Ocean. The Yangtze River forms a watershed south of the divide, emptying into the East China Sea. Although the Yellow and Yangtze Rivers never meet, they both flow into the Pacific Ocean. A major divide that separates smaller watersheds can be found in the West African country of Ghana. All rivers in Ghana ultimately flow into the Gulf of Guinea in the Atlantic Ocean. The Volta River system drains about two-thirds of the country’s waterways. However, there are a number of smaller rivers, such as the Ankobra and Tano Rivers, that are not tributaries of the Volta. They flow independently into the Gulf of Guinea. Minor Divides A minor divide separates rivers that will later converge, or form a confluence. Most river systems have minor divides. North America's Missouri and Mississippi Rivers form a minor divide. The Missouri River's watershed extends as far north as the Canadian provinces of Alberta and Saskatchewan, and as far south as the U.S. state of Missouri. However, the Missouri is a tributary of the Mississippi River. The waters of the Missouri meet the Mississippi near Hartford, Illinois, and ultimately flow to the Gulf of Mexico. Navigating Divides Minor divides are not huge obstacles to navigation. Engineers on ships can usually navigate across a minor divide—the watersheds ultimately flow together. Major divides and continental divides, however, can be very difficult to navigate. Natural features can sometimes help explorers navigate major divides. The Chicago Portage, for instance, is a natural water gap formed through glaciation—the same process that created the Great Lakes. The Chicago Portage links the watersheds of the Great Lakes and the Gulf of Mexico, and is the key reason the city of Chicago became such an important shipping and industrial hub. Water gaps such as the Chicago Portage are rare, however. In most cases, engineers and geographers must rely on both watercraft and terrestrial transportation to navigate divides. Before the Industrial Revolution, explorers had to portage between major divides. To portage is to carry boats or other vessels across dry land. For large ships, portaging can be a long, difficult, and expensive journey. Smaller watercraft, such as canoes, are easier to portage than large ships. For this reason, Native Americans were more skillful at portaging around the rivers of North America than European explorers, who brought large, heavy sailing vessels. The Industrial Revolution changed the way sailors navigated. Engineers constructed dams and canals between divides. River management techniques controlled the amount of water flowing through parts of a river, and portaging became less necessary. The canals of the St. Lawrence Seaway are an example of industrial technology crossing a major divide. The canals make it possible for ships from the Atlantic Ocean to cross the divide separating the ocean from the watershed of the Great Lakes. Ships can efficiently transport both finished goods, such as shoes or cars, and raw materials, such as timber and iron, between the Great Lakes region and the rest of the world.