After rock has been loosened by weathering, it is worn away and moved to another place. The wearing away and moving of weathered rock and soil is called erosion. The main agents, or causes, of erosion are rivers, waves, glaciers, wind, and gravity.
River Erosion
(text found on page 286-291 in AGS Publishing: Earth Science)
Water running downhill is a powerful force. In fact, rivers and their tributaries change more of the landscape than any other agent of erosion. After rain falls to the Earth, the water flows downhill. The water pushes soil and rock fragments as it moves. These solid particles are sediment. The water and sediment flow into small gullies, which lead to rivers.
As water flows in a river, it erodes the banks and riverbed, which is the bottom of the river. Compare the eroding power of a river to a hose. The force of water from the hose can easily dig up soil and move it across a lawn. A jet of water may even chip away at a sidewalk. Similarly, river water erodes the land. Sand and stones in the river scrape against the banks and river-bed, causing more erosion. The boulders in the photo below have been worn smooth by fast-moving water and sediment.
The Life of a River Valley
As you run your finger through sand, your finger carves out a little valley. As a river erodes the land, it also carves out a valley. Some valleys are narrow with steep walls. THese are called canyons. Other valleys are wide and shallow. The shape of the valley largely depends on how old it is. Rivers and their valleys go through three stages: youth, maturity, and old age.
A young river is narrow and fast. Its swift waters rapidly cu down through rock, carving out a V-shaped valley. The river covers all or most of the valley floor. The fast waters have a lot of energy and can push rocks along the river’s path. Rapids and waterfalls are common. The Yellowstone River and Niagara River are examples of young rivers.
(from: ontariopics.com)
As erosion continues, a river becomes mature. At this stage, the boulders and rocky ledges that cause rapids and waterfalls have been eroded away. The slope, or angel, of the river is less steep, so the river does not flow as fast. It can move pebbles, sand, and mud, but not large rocks. The valley of a mature river is much wider than the river itself. The Ohio river and Missouri River are mature rivers.
All rivers have curves. Mature rivers usually have more of them. compare the rivers below. The water flows faster and pushes harder against the outside of each curve and erodes that bank faster. Water slows down on the inside of each curve, allowing sediment to settle and build up. This process creates large, looping bends called meanders.
(from: berkeley.edu)
As a mature river’s meanders grow, its floodplain also grows. A floodplain is the low, flat area that a river covers when it overflows its banks. Floodwaters leave behind rich soil and nutrients on floodplains.
The valleys of old rivers are broad and flat. By this time, the river has eroded its way down to near sea level. Old rivers tend to have enlarged meanders and more of them. As a meander continues to grow, it forms almost a complete circle. During a flood, the river may break through its bands and flow straighter. The meander is cut off and becomes a C-shaped oxbow lake. The Mississippi river, shown in the radar image, is an old river.
(from: photojournal.jpl.nasa.gov)
River Deposits
Sediment carried by the agents of erosion is eventually dropped in a process called deposition. For example, when a river slows down, it may drop, or deposit its sediment. Heavy particles, such as stones, drop out first. As the river slows down further, lightersediment, such as sand and clay, drops out.
(from: geo.uu.nl)
A river slows down considerably as it empties into a lake or an ocean. The place where a river flows into a larger body of water is called the mouth of the river. Sediment settles out at the river’s mouth. Eventually, the sediment builds up above the water level and forms a fan-shaped area of land called a delta. As the diagram below shows, a river usually branches off a it winds through the delta and empties. A delta provides rich farmland. Much of Egypt’s farmland, for example, is located on the fertile Nile River delta.
(from: news.nationalgeographic.com)
An alluvial fan is similar to a delta. It forms at the base of a mountain where a mountain stream meets level land.
Wave Erosion
Waves in an ocean or a large lake change the shoreline through erosion and deposition. As waves pound the shoreline, they hurl not only water but also bits of rock and sand agains the coast. These materials chip away at the rocky shore. Waves also force water into cracks in rocks along the shoreline. With each wave, the water presses against the sides of the cracks. The cracks get bigger, and pieces of rock split off.
This type of erosion formed the cliffs, towers, and other rocky shapes shown in the photo below. During storms, waves reach higher on cliffs and carve steep sides. Arches form when waves erode through a cliff. If the top of an arch collapses, a tower of rock called a sea stack is left standing.
(from: s0.geograph.org.uk)
Wave Deposits
Beaches are areas where waves have deposited sand, pebbles, or shells. Some of this beach material is sediment from nearby eroded rocks. Other beach material is sediment carried to the lake or ocean by rivers. Currents near the shore carry sediment to different parts of the shoreline. As waves break on shore, the sediment is pushed onto the beach.
Currents along the shore can change the shape of a beach. One result is a spit, or curved finger of sand, sticking out into the water. Waves and currents can also carry sand away from the beach and deposit a long, underwater sandbar offshore.
Geosystems 