Ephemeral streams as opposed to permanent streams

All waterways play a crucial role in providing an adequate supply of water to animals, humans, as well as vegetation. It even applies to the seemingly insignificant and little-known ephemeral stream.

An ephemeral stream is a temporary stream that only flows for a brief period as a direct result of precipitation, which mainly occurs in arid and semi-arid regions where rainfall occurs infrequently. It differs from intermittent streams by not having a clearly defined physical channel or bed.

Not all rivers and streams continuously flow throughout the year. Some of them are seasonal, and some only flow during specific events. No matter what their size or frequency, they all form part of the Water Cycle that distributes and cycles water around the planet.

One such temporary stream is called an ephemeral stream. It does not flow often and only for a short duration during specific occurrences. Yet, it is still just as an important source of water as any extensive permanent river system. 

This post examines what an ephemeral stream is, how it forms, and how it differs from other types of streams and rivers.

Ephemeral Stream Definition

Most major river systems and larger streams flow throughout the year, meaning they are perennial. (Like the Rhine in Europe, the Amazon in South America, and the Nile in Africa.) An ephemeral stream, though, is not only transitory but is not even a seasonal occurrence.

To be able to describe how and why this type of stream form, one needs to gain a clear understanding first of what precisely an ephemeral stream is:

An ephemeral stream is a temporary stream that only flows for a brief period as a direct result of precipitation. It occurs mainly in arid and semi-arid regions where rainfall occurs infrequently. It differs from intermittent streams by not having a clearly defined physical channel or bed.

The word "ephemeral" is derived from the 16th Century Greek word "ephēmeros," which aptly means "lasting for a very short time." And it is the brief occurrence of an ephemeral stream that is its most defining characteristic.

Since these streams occur for such a brief period, it does not have the time to carve out a deep and wide channel, as is the case with a perennial river. It also occurs in predominantly dry regions, where the groundwater table forms much deeper below the surface.

As a result of these two factors, ephemeral streams occur above the groundwater table, compared to more permanent streams and rivers, which riverbeds lie below the water table. The significance of the groundwater table height will become clear in a later section.

How Ephemeral Streams Formed

In arid or semi-arid regions, precipitation occurs very infrequently. As a result, when a large enough amount of rainfall does occur, it often forms a temporary stream on the surface.

The stream can create a new path or follow an existing channel (also called a dry wash) established by previous occurrences of ephemeral streams.

The paths these streams follow can link up with larger networks of intermittent or perennial streams and rivers.

They can also continue to flow for a short distance before evaporating completely without reaching any significant point or being absorbed into the soil to form groundwater.

Ephemeral streams flow for a limited time and dry up quickly, only leaving a dry stream bed behind. These dried-up channels are sometimes more accurately described as arroyos, which are synonymous with the arid and semi-arid areas where ephemeral streams occur. 

It is important to note that the ephemeral stream and the channel (arroyo) it flows in are not the same things. The stream itself is the actual flow of water that is transitory, while the channel it flows in remains a permanent fixture of the landscape.

Importance Of Ephemeral Streams

It is only natural to conclude that ephemeral streams play an insignificant role in contributing to the Water Cycle and have any other beneficial influences on the environment. Such a conclusion can not be further from the truth.

These streams play an essential role in supplying fresh and maintaining existing resources in at least three different ways:

1) Fresh Water Supply To Perennial Water Networks

Even though ephemeral streams only flow during or after a spell of rain, the combination and frequency of these streams have a huge impact. In fact, they contribute the vast majority of freshwater to the entire river network in arid and semi-arid regions.

For example, 95% of all streams in the Arizona Dessert are seasonal, of which a substantial amount are ephemeral. Even in wetter regions with frequent rainfall, it is estimated that more than 50% of the total stream network consists of temporary streams.

2) Supply Of Fresh Sediment To  Downstream Regions

During extended dry spells, dried-up stream beds (arroyos) builds up a layer of soil, which nutrient content hasn't been depleted by vegetation growth. 

The organic matter created by dead animals and insects, as well as the remains of dead plants, also accumulate in arroyos, further enriching the nutrient content of the soil.

When a spell of rain causes an ephemeral stream to flow, it carries this nutrient-rich soil downstream, where it gets deposited on riverbanks and the surrounding areas, replenishing the land with much-needed fresh sediment.

3) Maintenance And Replenishment Of Groundwater Tables

As the arid and semi-arid regions, where ephemeral stream occurs, don't contain much moisture, the groundwater tables are situated much further below the surface than in wetter areas with an abundance of rain.

When a substantial amount of rain falls, it allows ephemeral streams to contain a large enough volume of water to have some of it absorbed by the ground to replenish its deep water tables.

It can also flow far enough to connect with more permanent (perennial) river networks downstream. It not only supplies these systems with additional water but also assists in maintaining and replenishing their groundwater tables as well.

The Difference Between Ephemeral And Intermittent Streams

Some confusion exists among observers about the difference between ephemeral streams and intermittent streams since they are both regarded as temporary streams.

Ephemeral streams have already been clearly defined as temporary streams that only flow as a direct result of precipitation. The depth of their groundwater tables also means that they can't access this water source to sustain their flow in any way.

Intermittent streams, however, differ in more than one way. They are often seasonal, meaning that although they don't flow throughout the year, they receive a steady supply of water during the rainy season, which allows them to flow for sustained periods.

They also have deeper and more prominently defined river beds, combined with shallower groundwater tables as a result of the availability of more water. It allows the river beds to lie below the water table, allowing them to access groundwater to sustain their flow.

Conclusion

What became clear through this article is how a seemingly insignificant occurrence can play a significant role in a much more extensive network.

An ephemeral stream is not only temporary but only flows for a brief period during or after a spell of rain. Yet, in many regions, they account for the vast majority of water supply to major river networks, enabling them to flow throughout the year.

This article explained what an ephemeral stream is, how it forms, and its importance to larger, more perennial water networks. It also addressed and clarified the difference between an ephemeral and intermittent stream.

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Until next time, keep your eye on the weather!

Streams are bodies of water that contain banks and a channel and flow across the earth’s surface via gravity. Different characterization and classification methods exist. Streams are classified as a river further downstream when they have moved up within the Stream Order. This order organizes streams from 1st order streams to 12th order streams, growing in size and strength as their order increases. The Rosgen Stream Classification system assesses different patterns of channel morphology to place streams within this order. Though this system seems like a classification of natural rivers and streams that can be easily performed, there are many specific details that go into this process.

First-order streams are the smallest and are called tributaries as no water flows into them. These waters eventually meet to form rivers, taking on the classification of the most significant contributing stream. Tributaries typically exist on steep slopes and maintain fast streamflow. The development of flora and fauna in these channels is unlikely. These streams eventually meet other order waterways and slow down. First to third order streams make up 80% of the world’s waterways and are commonly referred to as headwater streams.

Downstream from these waters are fourth to sixth order streams and can be described as medium streams. Sixth order streams to twelfth order streams are the largest of these waters and are called rivers. Medium and Larger streams’ slope rests at a lower grade, creating slower streamflow.

Perennial and Intermittent Streams

What is a Perennial Stream?

A Perennial Stream (or perennial river) is categorized by year-round stream flow in parts of its stream bed. This assessment assumes normal rainfall, as climate change impacts how these streams are classified. These streams are called a “permanent stream” because they are clear and continuous compared to a “temporary stream,” such as ephemeral and intermittent streams. Perennial streams usually occur downstream and lack extensive vegetation. The baseflow present downstream is large enough to sustain perennial streams and provides a consistent water flow that hinders the development roots.

What is an Intermittent Stream?

Intermittent streams (or intermittent rivers) have streamflow for only a portion of the year. These streams have a well-defined channel and are often called a “seasonal stream.” Intermittent streams may not have streamflow during the dry months (especially in arid regions) as they rely on present groundwater and precipitation runoff to provide their streamflow. The dry period is the main distinguishing characteristic used for intermittent and perennial stream classification.

What is an Ephemeral Stream?

Ephemeral Streams should be included in this type of classification. They remain dry for a portion of the year and only have flowing water after precipitation. These shallow waters lack a defined stream channel and rest above the water table year-round. Ephemeral streams rely on stormflow for their current and will likely not present characteristics similar to a perennial stream until sufficient precipitation occurs.

Other Types of Stream Classifications

Alluvial Fans

Alluvial fans are waters exiting a steep region and entering a flat area. They have an outward flow, classifying them as distributaries. These waters deposit sediment at the base of their stream channel once they arrive on flat land. These fans accumulate a significant sediment load as they travel through canyons towards their final destination.

Braided Streams

The main characteristics of braided streams are channels that consistently branch off and rejoin. Braided streams receive their name from the appearance of braiding hair created by this movement. This “braiding” process produces a buildup of sediment called an anastomosing. This feature exists between the channels, appearing in long bars rather than fans and triangular formations seen in alluvial fans or deltas. These streams flow near high mountain ranges, like rivers crossing near the Rocky Mountains.

Deltas

Deltas are distributary channels that form from a single track entering a large body of water, such as a sea. The source rivers will subdivide into smaller streams before entering the larger body of water. Deltas form from the buildup of sediment deposited from the stream into the more significant water and will eventually fill standing bodies of water with sediment. They receive their name from their standard triangle shape, though they may appear in other forms. Famous examples of deltas are the Mississippi Delta and the Nile Delta.

Meandering Streams

These streams are found in relatively flat areas, looping and twisting across the large floodplain. They serve as erosional waters and occasionally depositions waters, yielding a high energy-to-load ratio. This erosion causes them to laterally grow as they deposit sediment outside the bends of these loops, surrounding them with mud, silt, and sediment. The sinuosity of these streams can alter sediment transport and deposition. Eventually, if these bends increase to be too large, the stream will connect these loops as means to a more efficient flow. This geomorphic process creates an oxbow lake.

Straight Channel Streams

Straight Chanel Streams lack the winding seen in other stream types listed prior. They follow a single channel with valley walls that steeply enter the water, negating the occurrence of a floodplain. These types of stream channels are typically found in canyons and serve for erosional work. As these waters occur in areas with high ridges near the head of a river, the strong flow quickly transports sediment. An example of this type of stream would be the Colorado River in the Grand Canyon.

Headwater Streams 

Headwater streams are small tributaries considered the highest end of a watershed. Headwater streams will feed into larger rivers and are the smallest parts of river networks. Additionally, these streams represent the majority of all cataloged streams worldwide. Headwater streams can be ephemeral, intermittent, or perennial but are typically small in nature. Plants and wildlife are generally abundant in and around these streams, and they are vital to rivers because they provide flow, sediment, and organic matter.