How to Read River Basin Maps

You've got a river basin map in front of you, or you're thinking about getting one. Either way, you're probably wondering: what exactly am I looking at? River basin maps are deceptively rich. However, once you know what the lines, colours, and shapes represent, they stop being merely decorative and start telling a story. Here's how to read one.

What You're Actually Looking At

Let's start with the big picture. A river basin (also called a watershed, catchment area, drainage area, or drainage basin... yes, there are that many names) is any area of land where precipitation collects and drains into a common outlet: a river, a lake, or the sea.

Think of it as a giant funnel. Rain falls, finds the lowest path, and eventually joins a river. Everything within that funnel is the river basin.

On our maps, each river basin has its own distinctive colour. The map of the contiguous United States is a great example: all those interconnected pink lines belong to the Mississippi catchment, which is the fourth largest in the world.

Now, here's where it gets linguistically interesting. In North America, watershed and river basin are used interchangeably. Outside North America, watershed actually refers to the elevated terrain surrounding the basin: the ridges, hills, and mountains that act as the dividing line. Same word, opposite meaning. English linguistics never fails to amaze.

The Borders: Drainage Divides

The edges of a river basin are defined by a drainage divide (also called a water divide, ridgeline, or water parting - in every part of the world). This is the high ground, the spine of hills or mountains, that separates one basin from the next. Rain falling on one side flows into one river system; rain on the other side flows into a completely different one.

On our river basin maps, the drainage divide is the black space between two colours.

The elevation map of Ethiopia shows the Ethiopian Highlands in red. The mountain ridges are clearly traceable on the river basin map: check the line between the purple and the green catchments.

Following the Water: Tributaries and Confluences

Inside the basin, water flows from smaller streams into larger ones. Those smaller streams are called tributaries or affluents.

Where two rivers meet and merge, that's a confluence. On a detailed river basin map, confluences are often the most visually striking points. The branching tree-like structure of a river network becomes clear when you trace each tributary back to where it joins the next.

The confluence of the Kura and Aragvi rivers in Georgia.

Where the River Ends: Deltas and Estuaries

At the other end of the journey, most rivers meet the sea. When they do, they do it in one of two ways:

A delta forms when the river splits into multiple smaller channels (called distributaries) as it deposits sediment at its mouth. The Nile, the Lena, the Mississippi all have classic deltas.

An estuary is the broad, tidal lower course of a river where it opens directly into the ocean without splitting. The River Geba and the Thames are good examples.

On a river map that is based on satellite data, this distinction is usually not too prominent. Satellite images or drone photos are more suitable to highlight their beauty.

The Lena Delta Reserve in Russia and the estuary of the Geba River on the west coast of Guinea-Bissau. Satellite images by the USGS.

Not all rivers reach the sea

Some rivers flow into what's called an endorheic basin: a closed drainage system with no outflow to the ocean. Instead of reaching the sea, the water collects in an inland lake or swamp and simply evaporates. The Caspian Sea, the Dead Sea, and the Great Salt Lake are all examples of endorheic lakes (also called terminal lakes). Because nothing drains out, any minerals or pollutants carried in by rivers accumulate over time, which is why these lakes tend to be salty, and why they're particularly sensitive to environmental change. On a river basin map, an endorheic basin is a closed system: the drainage divide forms a complete ring, with no outlet to another river or the sea.

Endorheic basins with lakes in the state of Utah, United States.

Quick-Reference Glossary

For the full glossary, visit our River Basin Glossary.

Ready to put this to use? Browse our river basin maps and see the world's great river systems in a new light.