How are erosion and deposition by a stream related?

Many small brooks merge into larger streams through creeks, which in turn merge into rivers. Streams can flow into rivulets or rivers, and their volume can vary depending on evaporation and side-seepage.

The ground's slope influences the rate at which water flows because of gravity's pull on a steeper slope, which causes water to flow more quickly than on a more leveled surface; any depressions or ruts with steeper slopes cause water to collect in channels that eventually merge into deeper, bigger streams.

Extremely strong river currents can carry away boulders and cobbles, while weaker currents can only carry away smaller particles like pebbles, granules, and sand grains. The larger particles become stuck in the river bed when the current is unable to move them any further. Large or sudden water flows erode and carry away soil and rock particles (called sediments).

The smallest particles, silt and clay, remain suspended in the water and are carried onward for very long distances, even as far as the mouth of the river or out to sea; weak currents are unable to keep grains and mud suspended, so these drop down to the river bottom and form a layer of sediment.

A river that has deposited a lot of sediment may eventually slow down and deposit most of the sediment on the river bottom, making the river shallower at that point and allowing the deposited sediment to get very thick. Alternatively, if the river reaches flat land, it may overflow its banks on one side or the other and deposit levees, which are piles of deposited sediment.

Some rivers flow out into very flat land and form 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. Eventually, the river could get blocked by a natural or artificial dam which will stop the flow temporarily until it can overflow again.

Erosion by a stream involves the wearing away of rock and soil from the streambed and banks, while deposition involves the settling out of sediment carried by the stream. Erosion can lead to increased sediment load, which in turn can lead to increased deposition downstream when the stream's energy decreases.