A waterfall is formed when a layer or hard rock lies on top of a layer of soft rock. As the river passes over the hard rock, the soft rock below is eroded quickly to form a step in the river bed. A plunge pool forms at the base of the waterfall and over time the overhanging rock collapses into it.

The fallen rocks crash into the plunge pool . They swirl around, causing more erosion. Over time, this process is repeated and the waterfall moves upstream. A steep-sided gorge is formed as the waterfall retreats.

Waterfall and gorges A waterfall is a sudden drop along the river course. It forms when there are horizontal bands of resistant rock (hard rock) positioned over exposed, less resistant rock (soft rock). A steep-sided valley is left where the waterfall once was. This is called a gorge .

Waterfalls commonly form where water rushes down steep hillsides in upland areas. They are typical of the upper valley but can be found in the rivers lower courses where the process of rejuvenation creates enough potential energy for vertical erosion to recommence closer to the mouth of the river.

As a river reaches flatter land, it swings from side to side, forming winding bends called meanders. This causes the meanders to grow bigger and bigger over time. Eventually the river may take a short cut, cutting across the narrow neck of the loop, leaving a separated U-shaped lake known as an oxbow.

They are formed in two ways: When the slope of a river bed drops down all of a sudden, the water plunges down from the mountain in the form of a magnificent waterfall. Faulting may also result into a break in the land surface and hence, the slope of the river bed drops and results into waterfall.

The Work of Rivers. The erosional work of streams/rivers carves and shapes the landscape through which they flow. Load carried by a river will grind against its bed and sides. This process slowly wears the bed and sides away.

What is a delta? A delta is a feature formed when rivers drop off sediments in low-lying areas, usually as they enter the ocean, sea or an estuary.

This material is called alluvium or silt. Silt is rich in nutrients that help microbes and plants—the producers in the food web—grow. As silt builds up, new land is formed. This is the delta.

Estuary is an area where salt water of sea mixes with fresh water of rivers. Delta is a low triangular area of alluvial deposits where a river divides before entering a larger body of water. Estuary: It is formed by a tidal bore, which ablates the riverbed and carries the silt out to sea.

Sometimes river valleys are very steep and narrow, more like the letter “I” than the letter “V.” These narrow valleys consist of hard walls made of rock that the river has carved out from under the top layer of soil. The Mississippi River forms a massive delta in Louisiana where it meets the Gulf of Mexico.

In the upper course of a river most of the erosion is vertically downwards. This creates steep-sided V- shaped valleys. Watch the clip below explaining the formation of V-Shaped Valleys.

10 Wonderful Examples of V-shaped Valleys

• Grand Canyon of the Colorado River.
• Yosemite Valley.
• Iao Valley in Hawaii.
• Muretto Pass in the Swiss Alps.
• Black Canyon, Gunnison National Park, North America.
• Upper Inn Valley (Inntal), Austria.
• Napf region, Switzerland.
• Zurich Oberland, Switzerland.

What happens when a stream slows down? The stream picks up larger particles. The stream has more energy for erosion. The stream picks up more sediment.