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Runoff and Erosion Moving water is the major agent of the erosion that has shaped Earth's land surface. As water moves over the land, it carries particles with it. This moving water is called runoff. Runoff is the water that moves over Earth's surface. The amount of runoff in an area depends on 5 main factors. 1) The amount of rain an area receives. 2) Vegetation of the area. Grasses, shrubs and trees reduce runoff by absorbing water and holding soil into place. 3) The type of soil. Some types of soils absorb more water than others. 4) Shape of the land. Land that steeply sloped has more runoff than flatter land. 5) How people use the land. For, instance a paved parking lot absorbs no water , so all the rain that falls on it becomes runoff. More rain means more erosion.
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Rills, Gully, Stream and Tributary
Rills are the tiny grooves in the soil runoff makes. When many rills flow together it becomes a gully. A gully is a large groove or channel in the soil that carries runoff. When gullies come together they form a bigger channel called a stream. A stream is a channel with a continually flowing water down a slope. A stream gets bigger with tributaries. A tributary is a stream or river that flows into a larger river. An example of a tributary is the picture to the right. -->
Erosion by Rivers
This is an example of a tributary.
Waterfalls occur where a river meets an area of rock that is very hard and erodes slowly. Softer rocks wears away faster than harder rock. Rapids are areas of rough water also, occur where a river tumbles over hard rock. A river usually flows over more gently sloping land.The flat, wide area of land along a river is a flood plain. A meander is a loop-like bend in the course of a river. Sometimes a meandering river forms a feature called an oxbow lake. An oxbow lake is a meander that has been cut off from the river.
Deposits by Rivers
This is an example of a delta.
As water moves, it carries sediment with it. Any time water slows down, it stops, or deposits, some of the sediment. When a stream flows out of a steep, narrow mountain valley the water slows down. Here sediment is deposited in an alluvial fan. An alluvial fan is a wide, sloping deposit of sediment formed where a stream leaves a mountain range. When sediment deposited where a river flows into an ocean or lake builds up a land-form called a delta. Deltas can be a variety of shapes. 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 flooded plains.Deposition of new soil over a flooded plain is what makes a river valley fertile.
Groundwater Erosion
This is an example of groundwater erosion.
Groundwater is the term geologists use for this underground water. Groundwater can cause erosion by the process of chemical weathering. The action of carbonic acid on limestone can also result in deposition. A deposit that hangs like an icicle from the from the roof of a cave is known as a stalactite. Slow dripping builds up a cone-shaped stalagmite from the cave floor. If the roof of a cave collapses because of erosion of the erosion of the underlying limestone, the result is a depression called a sinkhole. This type of landscape is called karst topography.