Work and Energy
A river's water has energy. Energy is the ability to do work or cause change. There are two kinds of energy. Potential energy, and kinetic energy. Potential energy is energy that is stored and waiting to be used later. Kinetic energy is the energy an object has due to its motion. As gravity pulls water down a slope, the water's potential energy turns into kinetic energy that can actually do work.
When energy does work, the energy is transferred from one object to another. A river is always moving sediment from the mountains to the sea. At the same time, a river is also eroding its banks and valley.
How Water Erodes
Gravity causes the movement of water across Earth's land surface. But how does water cause erosion? During the process of water erosion, water picks up and moves sediment. Sediment includes soil, rock, clay, and sand. Sediment can enter rivers and streams in a number of ways. Most sediment washes or falls into a river as a result of mass movement and runoff. Other sediment erodes from the bottom or sides of the river. Wind may also drop sediment into the water.
Abrasion is another process by which a river obtains sediment. Abrasion is the wearing away of rock by a grinding action. Abrasion occurs when particles of sediment in flowing water bump into the streambed again and again. Abrasion grinds down sediment particles. For example, boulders become smaller as they are moved down a stream bed. Sediments also grind and chip away at the rock of the stream bed, deepening and widening the stream's channel.
The amount of sediment that a river carries is its load. Gravity and the force of the moving water cause the sediment load to start to move downstream. Most large sediment falls to the bottom and moves by rolling and sliding. Fast-moving water actually lifts sand and other small sediment, and carries it downstream. Water can dissolve some of the sediment completely. The river now carries these dissolved sediments in solution.
Erosion and Sediment Load
The power of a river to cause erosion and carry sediment depends on several factors. A rivers slope, volume of flow, and the shape of its stream bed all affect how fast the river flows and how much sediment it can erode.
A fast-flowing river carries more and larger particles of sediment. When a river slows down, it drops its sediment load. The larger particles of sediment get deposited first.
Slope
Generally, if a river's slope increases, the speed of the water does to. A river's slope is the amount the river drops toward sea level over a given distance. If a river's speed increases, its sediment load and power to erode may increase. But other factors are also important in determining how much sediment the river erodes and carries.
Volume of Flow
A river's flow is the volume of water that moves past a point on the river in a given time. As more water flows through a river, its speed increases. During a flood, the increased volume of water helps the river to cut more deeply into its banks and bed. When a river floods, its power to erode may increase by a hundredfold. A flooding river can carry huge amounts of sand, soil, and other sediments. It may move giant boulders as if they were pebbles.
Streambed Shape
A streambed's shape affects the amount of friction between the water and the streambed. Friction is the force that opposes the motion of one surface as it moves across another surface. Friction, in turn, affects a river's speed. Where a river is deep, less water comes in contact with the streambed. Therefore friction increases, reducing a river's speed.
A streambed is often full of boulders and other obstacles. This roughness prevents the water from flowing smoothly. Roughness thus increases friction and reduces the river's speed. Instead of moving downstream, the water moves in all directions in a type of movement known as turbulence. Friction and turbulence slow the stream's flow. But a turbulent stream or river may have great power to erode.
Factors Affecting Erosion & Deposition
The way a river flows depends on how it was affected by erosion and deposition.
If a river flows in a straight line it means that the water flows faster near the center of the river than along its sides.
When a river curves it means that the water moves fastest along the outside of the curve. The river cuts into the bank causing erosion. Sediment is then deposited on the inside curve where the water speed is the slowest.
If a river flows in a straight line it means that the water flows faster near the center of the river than along its sides.
When a river curves it means that the water moves fastest along the outside of the curve. The river cuts into the bank causing erosion. Sediment is then deposited on the inside curve where the water speed is the slowest.
The video above is a video on the factors affecting erosion and deposition.