Streams form when downhill running water creates a channel capable of carrying water. Most streams begin as temporary rivulets that only have water after a rainfall.
While a stream is still an intermittent waterway, water flow during and shortly after rainfall creates a small channel in a downhill slope that carries water. With the passage of time and continuous rain events, the stream's channel becomes deeper. When the channel is deep enough, it collects enough groundwater to keep it full between rain events and becomes a permanent stream.
When streams form on opposite sides of a slope, weathering and erosion cause a low area, or saddle, to form. In situations in which one side of the slope is steeper than the other, one stream erodes faster and cuts into the saddle to form a pass. Mountain passes provide a natural route for travel in mountain ranges.
The channels of young streams often have steep sides and rough, steep topography, which leads to waterfalls and rapids. As the stream ages, weathering and erosion smooth the channel, and the stream widens and becomes still. Older streams tend to have wide, gentle curves, or meanders, due to the force of water against the channel walls.