Streams and rivers cause erosion in two ways.
The force of running water cuts a channel into the earth, sometimes forming valleys or steep gorges. Then, running water picks up sediment—particles of earth called silt—and carries it away from its original spot.
When a stream has cut a path through the land, it creates a new destination for run-off from the surrounding terrain. Streams and rivers transport silt from their origin and gather more along their journey toward the ocean.
A new stream will form a relatively steep, V-shaped channel in the earth. It may contain waterfalls and rapids. Over time, with more and more running water, the channel will smooth out and become a gentler, wider U shape.
The stream will create a floodplain, a low-lying area that is subject to flooding when the stream overflows, with the actual water flow running in a channel through it. As streams age, they tend to wander, or meander, in loops through their floodplain. A stream can he identified as old by the presence of extensive meandering.
When the stream reaches the ocean, it may form a delta. A delta is a fan-shaped deposit of silt from the river. Deltas result from the running water dropping its load—the silt it is carrying—into the ocean.
The load is dropped because the stream’s water slows when it reaches the larger ocean, allowing gravity to force the silt to settle.