What is a Tributary? A Full Guide

Rivers are complex bodies of water that have many parts. Tributaries are critical portions of a river, and they have various levels of classification. So, what is a tributary? We’re going to explore this question in-depth, provide examples of tributaries you may know, and tell you how these systems are ranked. By the time we’re finished, you’ll know all about tributaries.

What is a Tributary?

A tributary is a body of water, usually freshwater, that feeds into a larger body of water without flowing into the ocean. For example, tributaries may be streams or rivers that feed into larger streams and rivers. As the name suggests, tributaries increase the flow of water to another body, offering their water as “tribute.”

Tributaries are especially common around large rivers. Often, many streams of water come together as tributaries for a large body of water. For example, the Mississippi River is believed to have over 200 tributaries that flow into it, including the longer Missouri River.

The concept of a tributary should not be limited by the idea that a large river can’t flow into another large river. Let’s take a look at some significant tributaries to better understand them.

Examples of Important Tributaries

The Mississippi River is a river that starts in Minnesota and runs the full length of the country until it reaches the Gulf of Mexico. However, the Mississippi River is not itself a tributary because it flows into the Gulf of Mexico in the Atlantic Ocean.

The Mississippi River is over 2,000 miles long, and two other very large rivers, the Missouri River and the Ohio River feeds into the mighty Mississippi River. Since those rivers feed into the main stem, offering their water to the long river, they are tributaries. Combined, those three rivers make the largest river system in the United States and one of the biggest in the entire world!

The Irtysh River is the longest tributary river in the entire world. This river measures over 2,600 miles and feeds into the Ob River. The Ob River is one of the longest rivers in the world and has a mouth in the Gulf of Ob, a bay of the Arctic Ocean.

These are just two examples of river tributaries. Oftentimes, rivers are fed water by very small streams that are hardly noticeable at all. As we’ll show you with the ranking systems, a collection of small streams and rivers can provide tremendous amounts of water to a river.

Important Tributary Terminology

As we have already mentioned, tributaries are bodies of water that feed into rivers. We’re going to take a look at some terminology surrounding these waters that will help your understanding of this term a little more.

Affluent

The term affluent is a synonym for a tributary stream. This definition is rarely used these days, but older research may contain this term and tributary interchangeably.

Confluence

A confluence is an area in which two bodies of water come together and continue as one. For example, the Mississippi River and Missouri River form a confluence near St. Louis. The Missouri River runs into the Mississippi River, feeding it water.  

Distributary

A distributary is defined as an area where a river branches off from its prominent run of water. You may see this occur as the river nears its mouth or a large delta.  

Drainage Basin

The drainage basin is a large area in which rainfall, snowmelt, groundwater, or other water collects and then forms into a river by way of many tributaries. The drainage basin contains many tributaries throughout hundreds or thousands of square miles, depending on the river the waters feed into.

Headwater

A river’s headwater is its point of origin and typically represents some of the smallest portions of the river system. The Mississippi River’s headwaters are found in Lake Itasca, a relatively small glacial lake in Minnesota. As the water flows south and more tributaries feed into it, the river grows from 20ft wide to several miles wide.

These definitions will help you better understand tributaries and their surrounding waters.

What Are Tributary Hierarchies?

When discussing tributaries, scientists often assign them values to determine their branching complexity. Also, they are used to denote a tributary’s importance to the flow of the river. Two major theories that apply to hydrology include the Strahler Stream Order and the Shreve Stream Order. Let’s take a closer look at each.

Strahler Stream Order

The Strahler stream order tracks the origin of bodies of water and represents a mathematical tree to quantify the complexity of branching segments. The Strahler stream order acts with three rules:

• If the node is a leaf (has no children), its Strahler number is one.
• When the node has one child with Strahler number i, and all other children have Strahler numbers less than i, then the Strahler number of the node is i again.
• If the node has two or more children with Strahler number i, and no children with a greater number, then the Strahler number of the node is i + 1.

This form of measurement is effective but complex for laypeople.

Shreve Stream Order

The Shreve stream order is a bit simpler to understand. All bodies of water that lack tributaries start at level 1. At confluences, places where the water flows into one another, the stream numbers are added. Thus, if every tributary is a “1”, it forms a “2” at the confluence. So, if the Missouri River, with a number of 3 formed a confluence with the Mississippi River that was numbered 4, then the resultant river would be a 7.

These are just two ways to identify stream orders and denote tributaries. These methods can be helpful in various studies of water. Identifying the tributaries of major rivers can help prevent pollution and give some insight into the history of the river.

Meanwhile, studying rivers is significant because of how dependent humans are upon them for economic and energy needs. Some rivers, like the Mississippi River, are poised to take new paths, and that could be devastating for the region. Although water flow is complex, it’s certainly worth understanding to some degree.

The photo featured at the top of this post is © iStock.com/Alx_Yago