Introduction to River

Riverb, a sizable stream of freshwater flowing through a natural channel in the land. Streams too small to be termed rivers are called by such names as brook, branch, and creek. Most rivers flow on the surface, but in some areas they go underground for great distances.

To humans, rivers have always been important physical features. Along such rivers as the Nile, Tigris-Euphrates, and Indus some of the greatest of the early civilizations were developed. During medieval times, riverbanks provided the sites for cities and towns, and river valleys became great avenues of commerce.

Rivers are even more important in modern times. They supply water for home and industrial uses. They provide cheap water power and transportation. They give needed water through irrigation projects. They aid in the disposal of waste. Also, they are often of great scenic and recreational value. Along their banks are some of the world's greatest cities. In their valleys are some of the finest agricultural lands.

Rivers are among the most powerful natural forces in shaping the earth's surface. In draining the land of surplus water, rivers wear down mountains, plateaus, and other high landforms. In a never-ending process, eroded material is carried by rivers. Some is deposited to form floodplains in the valleys, some forms deltas at the rivers' mouths, and some is deposited in the sea. Given enough time and no disturbances of the land, rivers could wear down all heights to almost sea level.

Rivers flow in all directions, the only limiting factor being that they follow the slope of the land. Most rivers ultimately flow to the sea, but many rivers lead to landlocked lakes and seas or dry up in arid wastelands. The Volga River is the best example of one flowing to a landlocked sea—the Caspian Sea. Most rivers in the Great Basin of the western United States and in Xinjiang province of western China dry up in arid lands.

Parts of A River

The bed of a river is the surface upon which it flows. Its banks are the sides that hold it in bounds. Right bank and left bank are terms frequently misunderstood. The right bank is on the right of an observer looking downstream (in the direction in which the river flows), the left bank on the observer's left.

A river begins at its source or headwater usually in a lake, spring, glacier, snowfield, swamp, or marsh—and flows to its mouth. A mouth is the point at which a river enters and ends in a larger body of water. The Mississippi, for example, has its source in Minnesota's Lake Itasca and its mouth in the Gulf of Mexico.

The course of a river is its path, which follows line of lowest elevation between the source and the end of the stream. Courses are often divided into three sections: the upper course, or that nearest the source; the middle course; and the lower course, that nearest the mouth. The channel, depending on how the word is used, is either the river's bed or the line of deepest water throughout the river's course.

River Systems

Just as a tree has a main trunk and many branches, a river system is made up of a main river and all its tributaries, or smaller streams. The point where two rivers join is called a confluence. In the Mississippi river system, for example, the confluence of the Mississippi and the Ohio, one of the principal tributaries, is near Cairo, Illinois. The entire land area drained by a river system is called a river basin, or sometimes a watershed.

Divide and water parting are terms used to designate the boundary along high land separating rivers and river basins. In the United States the Continental Divide in the Rocky Mountains separates the waters that eventually flow to the Atlantic and Pacific oceans.

Landforms Along Rivers

Rivers and running water are responsible for many of the earth's landforms. Valleys are elongated depressions, carved out over a period of thousands of years, through which rivers run. They become wider and flatter toward the mouth Canyons and gorges are deep and steepsided valleys formed by young rivers cutting mainly downward through the land. One of the largest is the Grand Canyon of the Colorado River.

In canyon rivers, as well as other places where the river bed drops quickly, swift and churning waters, called rapids, are produced. Waterfalls, the sudden falling of water from high ledges, are frequent occurrences on many rivers, especially those in mountainous regions. When large ridges of hard rock are cut through by rivers, water gaps are foumed;many are found in the Appalachian Mountains. A point is a wedge of land between two rivers that join.

Bordering some rivers are floodplains—broad, flat landforms built up by the sediments of successive floods. Rivers that are still cutting their valleys deeper and deeper do not build significant floodplains. Flood-plains are best developed along the lower courses of rivers. Here, too, along the banks, are naturallevees, which are the highest portion of the plain. Along some rivers, such as the Mississippi and the Huang He (Yellow River) of China, the levees have been built so high that the river bed is above the level of the surrounding land. Under such conditions, disastrous floods often occur. Meanders, or large winding loops in a river's course, are common on floodplains. Oxbow lakes are remnants of old meanders cut off from the river by a change of course.

At the mouths of some rivers, such as the Nile, Mississippi, and Ganges, are large landforms, usually triangle-shaped, called deltas. These have been formed by sediment from silt-laden rivers entering the relatively quiet, shallow water of the sea. The many branches of the river, through which the water discharges into the sea, are called distributaries. Along sunken coasts, the ocean backs up into the river's mouth to form estuaries. These V-shaped bays are often excellent harbors. Examples are the Hudson and Thames estuaries.

The Work of Rivers

Rivers perform a tremendous amount of work by erosion, the wearing away of the land's surface. All river water carries dissolved minerals and tiny particles of silt and clay. When the current of the stream is fast enough, it carries sand, gravel, and even boulders by suspension and by rolling them along the river bed. Thus the greater the speed and amount of water, the heavier the load the river can carry. Part of the material carried is washed into the stream; part is removed by the river itself from its banks and bed. In large rivers, such as the Mississippi, the amount of eroded material carried to the sea amounts to hundreds of millions of tons annually.

Rivers vary greatly in the amount of water carried. In regions of ample rainfall, they flow throughout the year. They receive water directly during rains; they have many tributaries; they receive ground water, which has seeped into the land during rains; and they are constantly fed by lakes, swamps, marshes, and other wet areas. The greatest volumes of water are carried by the Amazon River of South America and the Congo River of Africa. Both drain vast areas of rainy tropical land. In dry regions, rivers usually flow only after rains, which often come in downpours causing flash floods.

Rivers are of value in different ways throughout the world. In Southeast Asia, the fertile soils of the valleys of the Mekong, Salween, Irrawaddy, and Ganges support regions having some of the world's highest population densities. In Europe, the Rhine, Elbe, and Seine, and their many connecting canals, provide a fine system of river transportation. In North America, rivers are used extensively for transportation, water power, and irrigation. The major rivers of Africa, northern Asia, and South America are generally used less than rivers elsewhere because the lands through which they flow are sparsely populated.

Environmental Problems

Industrial, agricultural, and municipal wastes that enter rivers may cause disease in fish and other animals—including humans who drink contaminated water or eat animals that have absorbed toxic wastes into their tissues. The removal of trees and other plants from river basins increases the amount of soil that is carried by runoff into rivers. As accumulating soil reduces the depth of the rivers, shipping may be hampered and flooding may increase. Hydroelectric dams sometimes have adverse effects on wildlife; for an example,

In the United States, a number of federal and state laws help to reduce these environmental problems. Federal laws include the Wild and Scenic Rivers Act of 1968, which regulates such activities as damming and logging on and near certain rivers, and the Clean Water Act of 1972, which regulates the discharge of wastes into rivers and other bodies of water.