Rivers flow from their source, in hills or mountains, to the sea and in doing so they shape the land by eroding, transporting, and depositing silt, gravel, and other material. The flow or energy of the water depends on the gradient of the land over which it flows, remembering water always flows downhill, and how much water is in the river. The faster flowing the river, the more energy the water has, and the more it can erode its bed and banks, and transport that eroded material. When a river loses energy, it no longer has the energy to transport material and deposit it. The erosion rate varies from day to day and season to season.
Just as the sea crashing against the cliffs erodes or wears them away, so too the water movement, or energy, of rivers erodes their beds and banks. River erosion goes through many processes. Corrasion occurs when material carried in the water crashes into the river’s bed and banks breaking more material away. Particles carried along in the water knock together this process is attrition. Both corrasion and attrition are sometimes called abrasion. Corrosion is the process where the acids in the river water dissolve the rocks in the riverbed and banks as the water flows over them. Solution is the result of corrosion.
Hydraulic action occurs when the air trapped in crack and holes in the river is compressed, loosening material from the river’s banks and bed. Cavitation occurring mainly in waterfalls is the process whereby millions of tiny bubbles emit shock waves thus weakening the rocks.
The river transports material in many ways. Traction is the river’s flow rolling larger rocks and pebbles, which it is not strong enough to lift, along the riverbed. Dissolving materials by corrosion mean that the river moves eroded rocks in solution down the river. Saltation is particles jumping down stream. Small particles travel along the river in suspension.
As the river loses energy, it deposits material, heavy particles first. Lighter material continues its journey while the river has sufficient energy to carry it, eventually leaving the lighter material along the way, but the river may not drop the lightest material until it reaches its outlet to the sea. In this way, rivers sort material as they deposit it.
Rivers begin in hills, or mountains. Melting snow or ice may feed rivers or water draining down from high slopes into a basin. Each slope drains into a trickle or rill. Rills combine forming brooks, which then join together to form streams. Brooks and streams combine to form the tributaries of a river. When the river flows down from high hills and mountains, it flows through narrow steep gullies, which means that the river is fast flowing. As canoeists are well aware, in this stage, rivers change course rapidly and form dramatic features, white water rapids, waterfalls, and cascades. The rapid flow means high river energy, giving it the power to cut through rocks eroding them away.
The gradient of the land is not the only thing that affects the river’s flow, as runs towards the sea; friction is an important factor in its flow rate. The friction of the river against its banks and bed and friction between layers of water passing over and under one another affect the flow, as does the weather. One can see the difference when one looks at the same river after heavy rain and when one looks again after long periods without rain. Man’s actions too affect the rivers flow, gravel and water abstraction, irrigation and hydroelectric schemes and other activities can greatly alter a river’s flow rate.
As rivers leave their source mountains or hills, they look more like the familiar view of a river, wider, slower, and gentler, changing course more slowly. They form wandering S shaped meanders, these attracted many ancient humans, who recognized the added safety that a settlement surrounded on three sides by water could bring, and many towns e.g. Old Sarum, near Salisbury, originated from ancient settlement in a meander. Rivers still form valleys in this middle stage, but they are very different to the sharp mountain valleys, being deeper, wider, and more gentle. The point where two or more rivers join is a confluence.
The river carves from the high hills and mountains and carries it downstream, depositing it where it can build levees, which can keep the water level above the landscape level, such as the levees in the Mississippi. High water levels mean that water spills over the banks carrying mud and sediment and forming boggy flood plains, twisting and eroding the land in some places and building it up in others by deposition. Developers, who cover flood plains in houses and other concrete structures, create a time bomb, because severe flooding will happen one day. The flood plains of a river are often very fertile land because of deposition of silt. The Ancient Egyptians relied on the annual spring flooding of the Nile for their agriculture. There are often lush grasslands by rivers called river meadows.
In the lowest river courses, the ground is even less steep and the water runs more slowly still. The rivers reach the sea at estuaries, wide deep river mouths, or deltas. Deltas form when the river leaves sediment at its mouth meaning that the river splits into many narrow channels instead of one single river mouth. The river Thames in England, on which London stands, has an estuary. The river Nile in Egypt has a delta.
River processes shape the land in many ways and by many means. They erode, transport, and deposit silt, gravel, and other materials and by doing so they change the land. Rivers carve channels through rock. Rivers can make land fertile when they flood or bring misery to householders because man has built his settlements in the wrong place. It is no wonder that ancient man worshipped rivers or that still, today, rivers in some parts of the world are sacred. Even in modern cities, such as London and Paris, when people want a break from hectic modern life, they head to their river to sit, stare, reflect, and wonder at the river’s power.