Rivers shape their valleys by eroding and depositing sediments. The structure of the underlying rocks also plays a role, especially in determining the location of waterfalls and rapids.River Valley Formation

Longitudinal Profile

A graph of the slope of a river channel at each point along its course is called a longitudinal profile. In most cases, the slope of a river becomes less steep as it flows from its headwaters to the ocean. Slopes as high as 200 m/km (1000 ft/mi) can be found in mountainous terrain, but slopes of about 2 m/km (10 ft/mi) are more typical in the middle section of such valleys. Slopes as low as 2 cm/km (1 in/mi) can be found in rivers close to the ocean.

Through the dynamic interplay of erosion and deposition, most rivers develop a longitudinal profile that generally becomes less steep as the river flows from its headwaters to the sea. There are several reasons why the lower stretches of a river are usually less steep than the upper stretches and these reasons have to do with why the lower stretches of a river can still remove its sediment supply even with a shallower slope.

 An important factor is that the amount of water flowing in the river increases with each successive tributary that contributes to the flow. As the flow increases, a river is able to transport the same quantity of sediment with a shallower slope. A further factor is the tendency for the size of material being carried by rivers to decrease downstream as particles are weathered and abraded. As the average size of the particles gets smaller, a river is able to transport the smaller particles of sediment with a shallower slope.

Occasionally, the slope of a river changes abruptly along its course. Faulting or a transition from hard rock to soft rock along a river course can cause a sharp increase in the river slope. These increases in slope can lead to the formation of rapids or waterfalls, such as the Victoria Falls on the Zambezi River in central Africa. Sharp decreases in river slope can also be caused by faulting. If a river slope decreases abruptly, sediment will tend to be deposited at this point, which may lead to the formation of a fan-shaped accumulation of sediment called an alluvial fan. These features are

Deltas

Many valleys end in a delta, a fan-shaped accumulation of sediment where the river reaches the sea. Deltas form because the river supplies alluvium more rapidly than it can be removed by the action of waves and coastal currents. Notable examples are the delta of the Nile River, on the Mediterranean coast of Egypt and the delta of the Mississippi River on the Gulf of Mexico.

Glacial Valleys

Although most valleys owe their origin to erosion by rivers, other mechanisms can carve valleys in the landscape. In regions cold enough for ice to accumulate, glaciers can be a powerful erosive force capable of excavating spectacular valleys. Such glacial valleys typically have very steep sides and broad flat floors, giving a ’U’ shape cross-section compared with the ’V’ shape characteristic of mountainous river valleys. Most of the mountainous areas of North America and northern Europe have glacial valleys that formed during the last Ice Age. Glaciers flowed down river valleys in these regions, leaving steepened valley sides. Yosemite Valley in California is an example of a glacial valley with near-vertical valley walls.

Delta, deposit of soil or silt formed wherever a swift stream or river empties into a lake, ocean, or slower river, so called because its triangular shape resembles the Greek letter (delta). The term was first used by the Greek historian Herodotus in the 5th century BC to describe the Nile delta. The triangular shape and the great width at the base are due to blocking of the river mouth by silt, with resulting continual formation of distributaries at angles to the original course. Deltas are usually characterized by highly fertile soil. The combined delta of the Ganges and Brahmaputra rivers in India, and the deltas of the Nile River in Egypt and the Mississippi River in the United States are the world's largest.

Waterfall, sudden sheer descent of a stream or river over a steep drop in its bed, sometimes in a free fall. The term cataract, usually designating a series of rapids in a large river, is often applied to waterfalls of large volume. A waterfall of any height that has a small volume of water or is one of a series of falls is called a cascade. The term cascade is also applied to a waterfall if, while plunging, it maintains contact with the streambed. The highest waterfalls frequently are cascades. Waterfalls are valuable today as sources of hydroelectric power.

How Waterfalls Are Formed

Waterfalls develop in several ways. Different rates of erosion where a resistant layer of rock in a streambed overlies softer layers is the principal manner. Subsequent erosion of the softer rock by the falling water undermines and periodically breaks off portions of the harder cap rock. Some of the largest cataracts in the world, Niagara Falls in North America and Victoria Falls in Africa, for example, originated in this way.

 Waterfalls in mountainous regions generally develop where a glacier has deepened a major trunk valley, leaving less-eroded branch valleys hanging; the tributary streams in these valleys discharge into the main river by falls or cascades. Hanging valleys also develop where a main river deepens its channel more rapidly than its tributaries do. Such falls are among the highest in the world. Bridalveil, Ribbon, and Upper and Lower Yosemite falls in Yosemite National Park, Multnomah Falls (189 m/620 ft) on a minor tributary of the Columbia River in Oregon, Gavarnie (422 m/1385 ft) in France, Sutherland Falls (580 m/1904 ft) in New Zealand, and Staubbach Falls in Switzerland, for instance, all descend from hanging valleys.

Other waterfalls originate where a fault uplifts a mountain range or part of a range, creating a fault scarp over which streams drop steeply. Continued undercutting and erosion of the edge and of the rock bed above the falls move many waterfalls upstream; these ultimately diminish in size, dwindle to rapids, then disappear.