Tributary

Confluence of the Werra River with the Fulda River: both form the Weser River from that confluence.

In hydrology, a tributary corresponds to a watercourse, also called a tributary, which does not flow into the sea, but into another river that is usually more important, with which it joins at a place called the confluence.

General characteristics

Río Isábena, tributary of the river Ésera

In principle, of the two rivers that join, the one of lesser importance (due to its flow, its length or the surface of its basin) is considered the tributary. There are, however, many exceptions: the Mississippi River, whose tributary (the Missouri River) is, upstream of the confluence, 600 km longer and has a basin three times as extensive, for example.

Similarly, in Spain the cases of the Miño and the Nalón are shorter and less powerful than their tributaries, the Sil and the Narcea respectively. And at the confluence of the Orinoco with the Guaviare, the latter is longer (almost 1,600 km) than the Orinoco itself (940 km to said confluence), although it is not more mighty. These exceptions make us see that the name of the rivers is almost always a matter of toponymy, in which there is often no unequivocal logic about which is the main river and its tributary.

“Right tributary” and “left tributary”; or "tributary on the right bank" and "tributary on the left bank" are terms that indicate the situation of the tributary in relation to the flow of the main river. These terms are defined from the perspective of the waters of the latter in search of its lower slope, that is, in relation to the direction in which the fluvial course is running.

Ways to order them

In geography, the arrangement of tributaries is sometimes arranged from those closest to the source of the river to those closest to the mouth of the river. They can be ordered forming a hierarchy: those of first order, second, and third the most important. The first order tributary is usually the smallest in size. A second-order tributary is made up of two or more first-order tributaries, which combine to form the second-order tributary.

Another method is to organize the tributaries from the mouth to the source, in the form of a dendritic structure.

Finally, a way applicable to both methods is to also divide them by side: left or right, always from their head or source towards the mouth. This form of tributary classification has to do with the fluvial asymmetry of the rivers at the confluences of two or more of them. For example, Yazoo-type rivers are rivers that flow into the main river after running for a long time very close to its channel. They are very frequent in the Northern hemisphere on the left bank of the main river, which causes its mouth to form a very acute angle. On the other hand, the majority of rivers that are tributaries of another on the right bank of the main river almost always form a right angle and the few exceptions are due to the characteristics of the relief, which can modify this pattern.

Effluent

The opposite of a tributary is an effluent or distributary, that is, a derivation (natural or artificial) that emerges out of the main current of a larger river through a smaller one. Those of natural origin are found mostly in river deltas; although there are cases in which it occurs in other sections of the rivers, as it happens with the Casiquiare with respect to the Orinoco. Effluents of artificial origin are more frequent, that is, from a diversion, ditch or canal that is used for irrigation or water supply purposes in regions relatively far from the main river bed. For example, the case of the Mulholland Canal from the Colorado River to supply the Los Angeles metropolitan area. The Júcar - Turia canal is also an effluent of the former and is used to irrigate the orchards of Valencia and the supply of drinking water for this city.