Earth Science
Chapter 8
Section 3
A. Work and Energy:
As gravity pulls water down slope, the water’s potential
energy changes to kinetic energy that can do work.
Energy is the ability to do work or cause change.
There are two types of energy.
1. Potential energy – energy that is stored and waiting to be
used later.
2. Kinetic energy – energy an object has due to its motion, or
energy in motion.
B. How Water Erodes and Carries Sediment:
In the process of water erosion water picks up and moves
sediments.
Sediments include soil, rock, clay, and sand.
Most sediment washes of falls into the 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 grinding action.
Abrasion occurs when particles of sediment in flowing water
bump into the streambed, grinding down sediment particles.
This process also grinds and chips away at the streambed
deepening and widening the channel.
C. Erosion and Sediment Load:
A river’s slope, volume of flow, and the shape of its streambed
all affect how fast the river flows and how much sediment it can
carry.(Load)
The power of a river to cause erosion and carry sediment
depends on several factors:
1. Slope – If a river’s slope increases, then water speed
increases.
Slope is the amount the river drops toward sea level over a
given distance.
Slope = vertical drop/horizontal distance
If a river’s speed increases, its sediment load and power to
erode also increase.
2. Volume of Flow - A river’s flow is the volume of water that
moves past a point on the river in a given time.
More water means, greater volumes and increased speed.
A flooding river:
- increased volume helps the river to cut more deeply
into its banks and bed.
- may have hundreds of times more eroding power
than the river has at normal volume.
- Can carry huge amounts of sand, soil, and other
sediments, called its load.
3. 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 over another.
Friction affects a river’s speed:
Where a river is deep, only a small fraction of the water is in
contact with the streambed.
This reduces friction and allows the river to flow faster.
In a shallow river, much of the water comes in contact with
the streambed.
Therefore friction increases, reducing the river’s speed.
The roughness of a river’s streambed prevents the river from
flowing smoothly. Instead of water flowing downstream it
move every which way. This movement is called
turbulence.
The shape of a streambed affects the way it erodes and
deposits sediments.
Where a river flows in a straight line, the water flows faster
near the center of the river than along the sides.
Deposition occurs along the sides of the river, where the
water movers more slowly.
If a river curves, the water moves fastest along the outside of
the curve. There, the river tends to cut into the bank.
Sediment is deposited on the inside curve, where the water
speed is slowest.