From its source, a river flows downhill. This is because of the Earth's gravity. Small trickles of water join up to form a stream. The amount of water in the stream increases steadily as more streams, called tributaries, join it. Eventually the stream becomes big enought to be a river.

A drainage basin is all the land that supplies a river and its tributaries with water. If you could look down on a river from above, you would see that it branches. This is called a drainage pattern. The shape of the pattern depends on rocks, soil, climate and the changes made to the river. Radial drainage happens when streams flow down from a central high point, such as a mountain top. Other rivers, such as the Amazon, form a pattern like the branches of a tree. This is called dendritic drainage.

Rivers and their tributaries can form many other types of drainage patterns. Parallel drainage happens when streams flow in valleys that are parallel to one another. This might be.because movements in the Earth, millions of years ago, made the rocks 'fold' into parallel lines. Trellis drainage is comnon where massive layers of sedimentary rock have 'slipped'. Streams flow in channels that are parallel to each other, and tributaries join the streams at right angles.