Side valley

A side valley and a tributary valley are valleys whose brook or river flows into a greater one.

Upstream, the valleys can be classified in an increasing order which is equivalent to the usual orographic order: the tributaries are ordered from those nearest to the source of the river to those nearest to the mouth of the river. A confluence is where two or more tributaries or rivers flow together.

In the orographic classification (order of rivers) the tributary river has order n+1, if n describes the primary (or main) river. A river which flows directly into the ocean (e.g. the English rivers Thames or Humber) has the orographic order n=1, the River Ouse n=2, the Wharfe n=3 and so on.

However, the term "side valley" is used rather for higher order valleys near the mountains (example above: the Pennines), for the lower valleys often do not show a drastic relief. The reason is a simple one: The "mainstem river" (into which the secondary river flows) passes much more water than its tributaries and therefore

The higher the order of a valley, the steeper usually are the hillsides resp. valley slopes. Looking upstream, the steepest slopes are normally near the source of a brook (with the exception of very hard rocks in downstream direction).

The estuary of broad rivers lies rather in flat regions (river flats) than of rivers in higher elevation. Therefore, the height difference of the tributary near its mouth is small (e.g. 1 m per km)—but much more at waters of higher order (in the Alps up to 100 m per km). This is one reason of the large number of hanging valleys in some mountain ranges (e.g. Salzburg or Graubünden).

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Wikipedia