Valley is a natural trough in Earth’s surface. Systems of valleys extend through plains, hills, and mountains. Rivers and streams flowing through valleys drain interior land regions to the ocean or to a smaller body of water, such as a lake. The bottoms of many valleys have fertile soil, which makes excellent farmland.
The bottom of a valley is called its floor. Valley floors slope downstream. Mountain valleys usually have narrow floors and steep walls shaped like the letter V. High valleys once occupied by glaciers have shapes closer to that of the letter U. In low-lying plains, a floor may be several miles or kilometers wide. The part of the valley along riverbanks often floods. This area is called the flood plain. When the river overflows its banks, it floods the flood plain. In some cases, flooding is helpful because it adds nutrients (nourishing substances) to the soil. But severe flooding can damage crops and buildings and even kill people. A valley’s sides are called valley walls or valley slopes. The ridge formed where the walls of neighboring valleys meet is a divide.
Kinds of valleys.
Various kinds of valleys are named according to their appearance. A deep valley with steep, often vertical walls is called a canyon. One of the most famous canyons is the Grand Canyon in Arizona. Along coastlines, valleys flooded by the ocean are called drowned valleys. Chesapeake Bay and Delaware Bay are drowned valleys.
Where a valley joins a larger valley from the side, the two floors usually meet at the same level. But sometimes the floor of the side valley is higher than that of the main valley. The side valley is then called a hanging valley. A river flowing through a hanging valley may form a waterfall where the water enters the main valley.
Not all valleys are on land. Many deep submarine canyons are found on the slopes leading up from the ocean floor to the edge of the continental shelf. Hudson Canyon is a submarine canyon. It extends south-eastward down the continental shelf to the Atlantic Ocean floor from a point near New York City.
How valleys are formed.
Most valleys on dry land are formed by the running water of streams and rivers, and by the erosion of slopes leading to them. Erosion moves material down the slopes to the valley floor where the river carries it to a lake or to the ocean. In addition, the stream may incise (cut into) and deepen its channel. Hanging valleys are usually formed when erosion is greater in the main valley than in the side valley. Also, areas eroded by glaciers often have hanging valleys.
A rift valley may form when a long, narrow section of Earth’s surface sinks as two pieces of the crust pull apart. One system of rift valleys extends from the Sea of Galilee in Israel south through the Red Sea and into southeastern Africa.
Glaciated valleys are valleys enlarged from erosion by glaciers. They are often found high in mountains and are U-shaped rather than V-shaped.
