Erosion by Water
Moving water is the major agent of erosion. Rain carries away bits of soil and slowly washes away rock fragments. Rushing streams and rivers wear away their banks, creating larger and larger valleys. In a span of about 5 million years, the Colorado River cut deeper and deeper into the land in what is now the U.S. state of Arizona. It eventually formed the Grand Canyon, which is more than 1,600 meters (1 mile) deep and as much as 29 kilometers (18 miles) wide in some places.
Erosion by water changes the shape of coastlines. Waves constantly crash against shores. They pound rocks into pebbles and reduce pebbles to sand. Water sometimes takes sand away from beaches. This moves the coastline farther inland.
The Cape Hatteras Lighthouse was built in 1870, on the Outer Banks, a series of islands off the coast of the U.S. state of North Carolina. At the time, the lighthouse was nearly 1,000 meters (3,300 feet) from the ocean. Over time, however, the ocean eroded most of the beach near the lighthouse. By 1999, the surf endangered the structure. Many people thought it would collapse during a strong storm. The lighthouse was moved 880 meters (2,900 feet) inland.
The battering of ocean waves also erodes seaside cliffs. It sometimes bores holes that form caves. When water breaks through the back of the cave, it creates an arch. The continual pounding of the waves can cause the top of the arch to fall, leaving nothing but rock columns. These are called sea stacks. All of these features make rocky beaches beautiful, but also dangerous.
This short animation explains how erosion causes a waterfall.
In rivers and in the sea, their are a number of different erosion processes at work:
Abrasion or corrasion – This is when large pieces of bedload material wear away the river banks and bed. This occurs when rock particles already being moved by the river strike or are dragged along the rock bed. The impact removes small particles. Small particles tend to smooth and polish the surface of the rock bed. Abrasion is thought to be the most effective method of erosion and is probably responsible for most of the down-cutting in a river channel. A stream with little load will not experience much abrasion. Most abrasion takes place during short violent periods of flow, such as in a flood. Potholes in a streambed are produced by the grinding action (abrasion) of a small number of hard pebbles.
Attrition – This is when the bed load itself is eroded when sediment particles knock against the bed or each other and break, becoming more rounded and smaller. This is the reduction in the size of particles in transport as they strike one another or the bed and banks of the channel. The consequence is that as particles are moved downstream, there is a steady reduction in size. In addition, the angular edges of the particles become more rounded, since they are more exposed. Attrition therefore contributes to the decreasing size and increasing roundness of particles with distance downstream although other factors are also important, particularly the sorting processes that take place as finer particles are more easily and more regularly transported downstream in periods of modest discharge. Large and angular particles are less likely to be moved, except in infrequent periods of high discharge.
Hydraulic Action – This is when the force of water erodes softer rock. This process moves material into the river channel through the impact of moving water, and its frictional drag, on the bed and banks f the river. Usually, only unconsolidated material, such as sand and fine gravel can be removed. The implosion of air bubbles in areas of great turbulence, such as in waterfalls, can cause shockwaves that can add to the erosion process.
Solution or corrosion – This is when acidic water erodes rock. This is most important on carbonate rocks such as limestone and chalk where carbonic acid (rainwater) dissolves the rock, which is carried away in solution.