Young streams have the strongest erosive powers. Both gravity and the narrowness of the water flow give them strength. As a stream gets older, the rock through which it has carved a channel will tend to erode. Sediment builds within the stream and causes it to flatten out more. As this happens, the water flow of the stream slows down, as does its ability to be a strong force of erosion. Steep, V-shaped valleys are signs of a young stream, while older rivers will not have very steep banks and will usually be part of a longer, wide river valley.

As rivers slow in speed, their ability to carry sediment decreases. Sediment tends to be deposited where the flow of water is the slowest. This is usually at the widest part of the river, where it meets a larger body of water. The pile of sediment at the mouth of a river or stream is called a delta. Deltas can be found where streams flow into lakes or oceans. As new sediment is deposited in the delta, the stream will regularly change its course. Eventually it makes a fan-shaped area of sediment, shaped like the Greek letter delta (Δ). The delta is where river water begins to mix with a body of water, such as a sea or lake.