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: Learn and explain how river valleys form 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