The uppermost part of a stream is called the headwater stream, and these can be just temporary (ephemeral), come and go (intermittent), or flow all the time (perennial).  Headwater streams often flow through wooded areas. They are shallow and cold, and usually carry leaves, twigs and other debris along with them. When the hills are steep, the streams run quickly and can begin to cut into the soil to form banks. Small soil particles are washed down the hill leaving larger rocks and boulders to form a stream bed. This is called the substrate of the stream. The combination of steep slopes and a more stable substrate means that the stream can form riffles and pools. Riffles occur when shallow, fast-flowing water runs over large rocks and boulders. Riffles can form anywhere along a stream, but are more common in the higher reaches, or headwaters. Water tumbles over large rocks to create waterfalls and hit the stream bed, cutting into the soil to form pools. These pools can also form behind blockages in the stream, or when the stream makes a sharp turn. The water in the pools is relatively deep compared to other parts of the stream and the water moves out of pools very slowly.

As the stream continues to flow down the slope, other small headwater streams join together to form a larger stream with larger banks.  This stream has more energy and can move more of the silt and soil from the stream bed.  The small streams (tributaries) come together to form a main stream, which typically becomes perennial.  This stream becomes wider and its bed contains a greater variety of material. It also begins to twist and turn, or meander. Where the channel of the stream is straight, it is referred to as a run. In this part of the stream, the bed often contains softer materials such as silt and sand.

The stream finally reaches the end of its journey when it flows into a river, a bay, or the ocean.