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Tributaries are the branches of a river, starting high up in a watershed and slowly working their way down to join other tributaries and eventually the mainstream, which drains to the sea. Tributaries come in a wide range of shapes and sizes, and the study of tributaries is a topic of interest for some biologists, because it is possible to use tributaries to follow pollution and various living organisms like invasive exotics back to their source. You can see tributaries on a map; look for the characteristic opening of a river on the sea, and then trace it inland. All of the small branches which appear are tributaries.
The word “tributary” comes from the same root as “tribute.” Both words come from the Latin tribuere, when means “to allot or distribute.” In a sense, tributaries could be said to be bringing tributes from the far reaches of a watershed to the river.
Rivers are designed to drain watersheds, areas of land which are surrounded by hills and mountains, creating a basin. Watersheds are also sometimes called “drainage basins.” The network of tributaries which combines to form a river is linked to numerous sources of water in a watershed, from areas which flood in the winter to glaciers which melt in the summer. Without tributaries and rivers, watersheds would slowly flood.
Some tributaries are little more than trickles in the land, while others are sizable rivers and creeks in their own right. All of them drain water, along with any substances which might have dissolved in the water, such as silt, decaying organic material, and pollution from places like farms, sewage treatment plants, and companies built along the banks of the tributary.
As tributaries meet up with each other, they become progressively larger, until they reach the main trunk of the river. Along the way, sediment settles out of the water, leaving a rich fertile layer of silt behind, along with any other materials which the tributary might have picked up. Eventually, the water reaches the ocean, where it will flow along with global currents, evaporate, and eventually start the cycle all over again.
Many nations have faced serious water pollution as a result of growing industrialization and human populations. Biologists often use tributaries to identify the sources of pollution; for example, if a river experiences an algae bloom, biologists might try to isolate the section of the river where the bloom is occurring before testing tributaries in that section to try and find the source of nutrients which is causing the bloom. Traveling along the tributary might reveal something like a leaking manure pit at a hog farm, or fertilizer runoff from a farm.