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Some of the most famous rivers in the world such as the Amazon and the Nile have been around for thousands of years and stay relatively constant from season to season. However, despite the fame of these year-round streams, rivers that come and go with the seasons are equally as interesting. These bodies of water are called ephemeral rivers and only flow after heavy rainfall or snowmelt. When there's not enough precipitation to start a current, these rivers exist only as dry channels that cut across the land.

Ephemeral rivers are beloved by ecologists since they're fascinating in terms of biodiversity and climate. If these dynamic ecological features have piqued your interest, here are three of the most important facts that you should know about them.

They Are Rich in Biodiversity

Ephemeral river with trees and mountains in Mongolia
Credit: kvv515kvv/ iStock

Ecologists measure biodiversity to determine the variety of plant and animal life that call a particular area home. It's often a helpful method to analyze how fertile or resource-rich a certain environment is, which can help other scientists study the area too. Ephemeral rivers boast incredible levels of biodiversity compared to regular rivers, which makes them a primary focus in numerous studies on aquatic life.

One such study published in 2019 by researchers in the U.S., France, and the U.K. dives into fascinating detail about what this biodiversity looks like and how it affects the health of the surrounding ecosystem. The researchers found that this dynamic living environment, where flow patterns change frequently, makes it much easier for organisms like fish and organic matter to move downstream. The result is that more land animals live by these rivers since they know they'll get an abundance of food and resources at certain times of the year.

This biodiversity isn't just about the number of organisms in the area — it's also about how unique they are. Aquatic animals that live in ephemeral rivers are particularly adaptive since they've developed a unique trait called desiccation resistance. In layman's terms, this means they can lose much more water than the average animal and not be negatively impacted. With the variability of ephemeral streams, this desiccation resistance is essential, which means a lot of nearby plants and animals have acquired this incredible ability.

They're Everywhere

Aerial view of bright blue river through a forest
Credit: GoncharukMaks/ Shutterstock 

Around the world, 50% of all precipitation drains into ephemeral rivers. Not only does this mean that these rivers are incredibly important to the world's ecosystem, but it also means that they're everywhere. Out of all the climates in the world, you're most likely to find an ephemeral river somewhere relatively dry. Arid environments such as the Sahara in Africa are prime candidates for these types of rivers since they have seasons of heavy rainfall and extreme dryness. However, locations don't have to be as extreme as a desert to house these ecological wonders. They could also be semiarid like the Mediterranean.

One of the most famous ephemeral streams in the United States is located at San Lorenzo Canyon in Socorro, New Mexico. There are also well-known rivers in the Gobi Desert in Mongolia, the Grand Coulee Dam in Washington state, and Providence Canyon in Georgia. Although these places are all very different in terms of landscape and climate, they each have ephemeral rivers, which is a testament to how diverse and interesting these bodies of water can be.

Since you can find ephemeral rivers all over the world, they have lots of different names depending on who you ask. In the United States, for example, people often call an ephemeral river an arroyo, gully, wash, or coulee. An arroyo, for example, is a Spanish loan word that's specific to rivers formed around slanted rock structures in the southwestern U.S. You'd only hear the term coulee, however, in the western U.S. and Canada.

Slight Rainfall Can Create an Ephemeral River

Mountain river stream valley scenery landscape
Credit: Ruslan Suseynov/ Shutterstock

When it rains in hot, dry regions, it rains hard. This is why ephemeral rivers are more commonly found in arid or semiarid environments. In this environments, the accumulated precipitation from a long timespan (potentially even an entire year) falls all at once. For example, the Sahara Desert only averages three inches of rain each year, but it may be years between rainy days.

This downpour of rain every once in a while is what forms ephemeral streams. When it's not rainy season, these arid environments slowly sap up all of the water from the river, leaving a dry bed. The river disappears altogether and leaves nothing but a channel of dirt where it once was. However, the next time it rains, the downpour of water quickly drains into that channel — filling it back up again until all of the water evaporates or seeps into the ground.

Another way that ephemeral streams are created is through several smaller channels coming together. Sometimes, instead of filling an old riverbed back up, the rain will create a new stream separate from the existing river. Over time, these channels tend to run together — making the river wider and wider with each new addition.

Once the rain stops, the river is only able to stay alive since it is constantly running. This motion prevents most of the water from soaking back into the ground and exiting the flow of the river. This is why ephemeral rivers last a lot longer after a heavy rain than puddles or other temporary bodies of water, where water is free to seep back into the ground immediately after it makes contact with the surface.