How are erosion and deposition by a stream related?
Erosion occurs when flowing water abrades and carries away soil and rock particles. When the current slows, the bigger particles fall out of suspension and deposit a layer of sediment on the bottom.
Many small brooks join together into a creek and then into a bigger stream. Many streams may flow into a rivulet or a river. Depending on the amount of water available, the stream can grow bigger or lose water through side-seepage and evaporation.
The slope of the ground affects the speed of flow of water because a steep slope and the attraction of gravity makes the water flow faster than a more flattened slope. Any depressions or ruts with steeper slopes makes the water collect in channels that gradually combine into larger and deeper streams.
Large or sudden water flows erode and carry away soil and rock particles (called sediments). Very strong river currents can carry away boulders and cobbles while weaker currents can carry away only smaller particles like pebbles, granules, and sand grains. The bigger particles get stuck in the river bed when the current can't move them anymore.
Weak currents can't keep grains and mud suspended so these drop down to the river bottom and form a layer of sediment. Silt and clay, the smallest particles, remain suspended in the water so they are carried onward for very long distances even as far as the mouth of the river or out to sea.
If a river carrying a large load of sediment suddenly reaches a flat slope, it eventually slows down and drops most of the sediment on the river bottom. This makes the river shallower at that point and the deposited sediment can become very thick. When the river reaches flat land, it might overflow its banks on one side or the other and deposit piles of sediments called levees.
Eventually, the river could get blocked by a natural or artificial dam which will stop the flow temporarily until it can overflow once more. Some rivers flow out into very flat land and forms a swamp, a marsh, or a delta (if formed near the mouth of the river). A few rivers in a hot climate lose so much water from evaporation that the stream stops flowing and is chopped up into small ponds on a drying riverbed.
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