Water Erosion

Runoff and Erosion

Moving water is the major agent of the erosion that has shaped the Earth's land surface. Water erosion starts with rain. Some rain goes into the ground, some into plants, and some evaporates. As the water moves around the land, it can loosen soil particles, and carry them with it. This process is called runoff. Runoff is water that moves over the Earths surface. If it moves in a thin layer, it could cause a type of erosion known as sheet erosion.

Amount of runoff

The amount of runoff in an area depend on five main factors. The first factor is the amount of rain that the area receives. The second factor is is plants/vegetation. Grasses, trees, and bushes reduce the amount of runoff by holding soil in place, and absorbing some of the water. The third factor is the type of soil in the area. Some types of soil can absorb more water than the others. The fourth factor is the shape of the land. The fifth factor is how people use, and treat the land. As an example, a paved parking lot doesn't absorb any water, so all of the rain that falls on it, becomes runoff. Runoff can also increases whenever a farmer cuts down crops because this removes vegetation.

The video to the right shows flowing water, to give an example of what rapids in a river do, and how they erode away sediment.

Rills and Gullies

Because Earth has gravity, Earth naturally pulls things toward the center of the Earth. So, gravity can cause runoff and the material it contains, to move downhill. As the runoff travels across the land, it starts to form very small grooves in the soil called rills. Over time, rills will start to into one another, they become larger, forming gullies. A gully is a large groove, also called a channel, in the soil that carries the runoff after a rainstorm.

The picture above is a rill.

The picture above is a gully.

Streams and rivers

Gullies join together to form a larger channel known as a stream. A stream is a channel along which water is continually flowing down a slope. Unlike gullies, streams barely ever dry up. If a stream is small, it can be known as a creek or a brook. As streams start to flow into each other, they join and start to make larger and larger bodies of flowing water. Usually, a large stream is called a river.

The picture above is a diagram of a stream.

The picture above is a picture of a stream.

The picture above is a picture of a river.

Tributaries

A stream grows into a larger stream or river by receiving water from tributaries. A tributary is a stream or river that flows into a larger stream or river. An example is the Missouri and Ohio rivers are tributaries of the Mississippi river. A drainage basin, or watershed, is the area from which water is collected from a river and it's tributaries.

The photos above are pictures of tributaries.

Erosion by Rivers

As a river flows from the mountains to the sea, the river forms many different types of features. Through erosion, a river can create features such as valleys, waterfalls, meanders, oxbow lakes, and flood plains.

Usually, rivers form on steep mountain slopes. Near its source, a river usually flows fast, and follows a straight, narrow course. The steep slopes along the sides of the river erode rapidly. This results in a deep, V-shaped valley.

Waterfalls

Waterfalls can occur when a river meets an area of rock that is very hard and erodes slowly. The river flows over this rock and then flows over softer rock downstream. In since the softer rock wears away faster than the harder rock, a waterfall will eventually develop where the softer rock was removed. Areas of rough water called rapids also occur where a river tumbles over hard rock.

The picture above is a famous waterfall known as Niagara Falls that is lit up at night.

The picture above is also a picture of Niagara Falls.

Flood Plain

Lower down on its course, a river usually flows over more gently sloping land. The river spreads out and erodes the land, forming a wide river valley. The flat and wide area of land along a river is called a flood plain. Usually, a river covers its flood plain when it overflows its banks during floods. On a wide flood plain, the valley walls may be kilometers away from the river itself.

The picture above labels the part next to the river known as a floodplain.

The picture above shows what a floodplain looks like.

Meanders

A river often develops meanders where it flows through easily eroded rock or sediment. A meander is a looplike bend in the course of a river. As the river winds from side to side, it tends to erode the outer bank and deposit sediment on the inner bank of a bend. Over time, the meander becomes more and more curved.

Because of the sediment a river carries,it can erode a very wide flood plain. Along this part of a river's course,its channel is deep and wide. Meanders are common. The southern stretch of the Mississippi River is one example of a river that meanders on a wide, gently sloping flood plain.

The picture above is a meander.

Oxbow Lakes

Sometimes a meandering river can create a feature known as an oxbow lake. An oxbow lake is a meander that has been cut off from the river. An oxbow lake may form when a river floods. During the flood, high water finds a straighter route downstream. As the flood waters fall, sediments dam up the ends of a meander. Now the meander is considered as an oxbow lake.

The picture above is an oxbow lake.

The picture above is a diagram of an oxbow lake as it forms.

Deposits by Rivers

As water moves, it carries sediment with it. Anytime moving water slows down, it drops, or deposits, some of the sediment. As the water slows down, fine particles fall to the river's bed. Now the larger stones quit rolling and sliding. Deposition can create landforms such as alluvial fans and deltas. It can also add soil to a rivers flood plain.

Alluvial Fans

Where a stream flows out of a steep, narrow mountain valley, the stream suddenly gets wider and more shallow. The water slows down. Here sediments are deposited in an alluvial fan. An alluvial fan is a wide, sloping deposit of sediment formed where a stream leaves a mountain range. As its name suggests, this deposit of sediment is shaped like a fan.

Deltas

A river ends its course when it flows into a still body of water, such as an ocean or a lake. Because the river water isn't flowing downhill anymore, the water slows down. At this point, the sediment in the water drops to the bottom. Sediment deposited where a river flows into an ocean or lake build up and create a landform known as a delta. Deltas can be a variety of different shapes. Some are arc shaped, others are triangle shaped. The delta of the Mississippi River is an example of a type of delta called a "bird's foot" delta.

Soil on Flood Plains

Deposition can also occur during floods. Then, heavy rains or melting snow cause a river to rise above its banks and spread out over its flood plain. When the flood water finally retreats, it deposits sediment as new soil. Deposition of new soil over a flood plain is what makes a river valley fertile. Dense forests can grow in the rich soil of a flood plain. This makes the soil perfect for growing crops.

The picture above is a picture of an alluvial fan is Death Valley, California.

The picture above is a picture of the Mississippi River delta.

The picture above is a picture of the Brazos River flood plain.

Groundwater Erosion

When rain falls and snow melts, not all of the water evaporates or becomes runoff. Some of the water soaks into the ground. There it fills the openings in the soil and trickles into cracks and spaces in the different layers of rock. Groundwater is the term geologists use for this underground water. Like running water on the surface, groundwater affects the shape of the land.

Groundwater can cause erosion through a process of chemical weathering. When water sinks into the ground, it combines with carbon dioxide to form a weak acid, called carbonic acid. Carbonic acid can break down limestone. Groundwater containing carbonic acid flows into any cracks in the limestone. Then some of the limestone changes chemically and is carried away in a solution of water. This process gradually hollows out pockets in the rock. Over time, these pockets develop into large holes underground, known as caves or caverns.

Cave Formations

The action of carbonic acid on limestone can also result in deposition. Inside limestone caves, deposits called stalactites and stalagmites can often form. Water containing carbonic acid and calcium from limestone drips from a cave's roof. Carbon dioxide is released from the solution, leaving behind a deposit of calcite. A deposit that hangs like an icicle from the roof of a cave is called a stalactite. Slow dripping builds up to forn a cone shape from the cave floor, called a stalagmite.

The picture above is an image of a cave or also called, cavern.

This is a diagram that shows you stalactites, and stalagmites.

Karst Topography

In rainy regions where there is a layer of limestone near the surface, groundwater erosion can significantly change the shape of the land. Streams are rare, because water easily sinks down into the weathered limestone. Deep valleys and caverns are common. If the roof of a cave collapses because of the erosion of the underlying limestone, the result is a depression called a sinkhole. This type of landscape is called karst topography after a region in Eastern Europe. In the United States, regions of karst topography are found in many states such as Florida and Texas.