Download 13.3 River Valleys

13.3 River Valleys
Key Idea:
The continuous caused by running
water tends to form V-shaped valleys
that grow longer and wider over time.
Objectives:
1. Learn and explain how river valleys form
2. Describe the formation of rapids and
waterfalls.
Headward erosion
 A single heavy rain may form a small valley in loose soil
along a hill slope. When the rain ends the \stream may
disappear, but the small valley remains. Such a feature is
called a gully.
 Gullies grow in length, width, and depth with every new
rain.
 The process by which land is worn away at the head of the
stream or gully is called headward erosion.
 If the valley grows in length and depth, the stream may cut
down far enough to become permanent. When the tributary
gullies also become permanent, then a river system is
born.
Gullies
Gullies and Headward Erosion
(Badlands National Monument, SD)
Canyons
A canyon is a river valley has very steep,
almost vertical sides.
Canyons are also called gorges or chasms.
Canyons form:
 In regions with little rain
 When the river cuts into its bed rapidly, due
to bed load erossion.
Grand Canyon
Gorge
V-Shaped River Valleys
 Most youthful river valleys are V-shaped.
Such valleys are found in regions where
there is enough rain to erode the sides of
the valley. As the river cuts its way down into
its channel, the upper valley walls are
widened into a V-shape by erosion.
How V-Shaped Valleys Form
 The valley forms gradually, by lateral erosion in
addition to the downward erosion of the channel.
V-Shaped Valley
The Base Level of the River
 The base level of a river is the level of the
largest body of water into which it flows.
 For streams that flow into the ocean, the
base level is the sea level. Sea level is the
ultimate base level, because ultimately all
rivers will eventually empty there.
 Lakes and rivers are local base levels for
streams that run into them.
Stream Piracy
 Steam piracy, or stream capture takes place
when through headward erosion one of the
rivers wears through the divide and captures
the headwaters of the other river.
 The first river grows larger and extends its
drainage basin at the expense of the
captured river.
Stream Piracy
Rapids and Waterfalls
 When the riverbed is steep enough, the river
forms whitewater rapids. It may level off
into a lake or pond, or the stream may
plunge over a cliff, to form a waterfall.
 Stream erosion is greatest at rapids and
waterfalls, because if the velocity (and
kinetic energy) of the water.
Erosion Created by Waterfalls
 One way a streams erode at waterfalls is by
undermining. Undermining happens when
water falling into the plunge pool at the base
of the waterfall erodes the rocks there,
leaving the locks at the top of the falls to
overhang.
 When pieces fall from the top, the waterfall
recedes, or moves upstream.
Waterfall
Niagara Falls
 Niagara Falls is a typical example of a
waterfall that recedes by undermining.
 The rocks underneath the waterfall consist
of alternating layers of resistant dolostone,
sandstone, and shale. The dolostone is
more resistant to erosion than the softer
shale at the base of the waterfall. As the
shale at the base of the waterfall is eroded,
the riverbed undergoes regressive erosion.
Niagara Falls Recessive Erosion
Diagram
Famous Waterfalls
 The highest waterfall in the world is Angel
Waterfall in Venezuela. Its height is 979 meters.
 Tugela Falls is the second highest waterfall (947
m). It is located in South Africa.
 Victoria Falls in Africa is considered the largest
waterfall in the world. It is 1.7 Km. wide, and its
maximum height is 106 meters.
 Niagara Falls is one of the largest waterfalls in the
world. It discharges over 6 million cubic meters of
water per second.
 Yosemite Fall is the highest in the US (1430 feet)
Angel Falls (Venezuela)
Victoria Falls on Zambezi River
Niagara Falls
Yosemite Upper Fall