Ch.9
Erosion and Deposition
Section 3: The Force of Moving Water
• Work and Energy
– Energy is the ability to do work or cause change
– Two kinds of 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 changes to kinetic energy that can do
work
– When energy does work, the energy is transferred
from one object to another
Section 3: The Force of Moving Water
•
How Water Erodes
– Most sediment washes or falls into a fiver as a result of mass movement
and runoff. Other sediment erodes from the bottom or sides of the river
– 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
– The amount of sediment that a river carries is its load
– Gravity and the force of the moving water cause the sediment load to
move downstream
– Most large sediment falls to the bottom and moves by rolling and sliding
– Fast—moving water actually lifts sand and other, smaller sediment and
carries it downstream dissolving some sediments completely
– The river carries these dissolved sediments in solution
Section 3: The Force of Moving Water
• Erosion and Sediment Load
– A river’s slope, volume of flow, and the shape of
its streamed all affect how fast the river flows and
how much sediment it can erode
– Slope
• 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
Section 3: The Force of Moving Water
• Erosion and Sediment Load
– Volume of Flow
• A river’s flow is the volume of water that moves past a
point on the river in a given time
• During a flood, the increased volume of water helps
the river to cut more deeply into its banks and bed
• A flooding river can carry huge amounts of sand, soil,
and other sediments
Section 3: The Force of Moving Water
• Erosion and Sediment Load
– 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 effecting the rivers speed
• Where the river is deep: less water comes in contact with
the streambed
• Where the river is shallow: much of the water comes in
contact with the streambed
• Roughness thus increases friction and reduces the river’s
speed.
• Instead of moving downstream, the water moves every
which way in a type of movement called trubulence
Section 3: The Force of Moving Water
• Erosion and Sediment Load
– Factors Affecting Erosion and Deposition
• Where a river flows in a straight line, the water flows
faster near the center of the river than along its sides
• If a river curves, the water moves fastest along the
outside of the curve, there the river tends to cut into its
banks, causing erosion
• Sediment is deposited on the inside curve, where the
water speed is slowest