A stream is a small, narrow river. It is formed near the source when trickles of water combine together.
As it flows quickly downhill, the water easily erodes (wears away) rocks to create a channel to flow in. Gravel, pebbles and boulders are then transported (carried) along and cause further erosion as they roll and bounce around.
V-shaped valleys are often cut into hillsides by streams eroding downwards.
If a river bed has alternate bands of hard and soft rock, it may erode into a series of ledges. This makes rapids form as the river surface breaks up into waves.