When two first order streams join, a second order stream is formed. Similarly, a third order stream is created by the joining of two second order streams, and so on. Where two streams of differing orders meet, the downstream portion retains the higher of those two orders. For example, second and third order streams would join and form a third order stream. It has been estimated that about 80 percent of Ontario's trout streams are first or second order streams.
Most scientists feel that a third order stream can be classified as a river.
The volume of water in a river is much greater than that of a stream and often a valley has been cut or eroded by a river's flow. The rate of flow in a river is usually much less than in a stream. The floodplain, or land periodically flooded by a river, becomes flatter, and silt and sand eroded upstream are often deposited on the bottom of the river bed as the river slows. The river's water tends to be turbid because of the suspended particles of clay, silt, finely divided organic and inorganic matter, etc. It is also often warmer from exposure to the sun and frequently contains less oxygen.