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.