Stream Classification

How are Streams Classified? 

Streams are classified using a variety of characterization and classification methods. They are identified from other bodies of water by their banks, channels, and flow across the surface. These features are then used to classify types of streams. For example, as a stream moves up within the Stream Order, the channel may grow, and flow might increase as the stream becomes a river. 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.

Why are Stream Classifications Important for Land Developers? 

These different types of stream channels may present as non-waters to developers at first glance. After precipitation, temporary streams may begin their seasonal flow and impact a developer’s project. The downstream effects these waters have and the ecosystems they impact are reasons for federal and state environmental regulation. These regulations can be costly for developers and negatively impact project timelines. Stream restoration after negative impacts can also drastically harm a project’s budget and timeline.

Without proper initial environmental due diligence, the steps taken to find environmental red flags on a site, developers can lose a large percentage of their budget on permits and required actions needed to develop near these protected waters. Permitting and recovery efforts may be necessary for projects extensively developed around protected environmental features.

How does the Clean Water Act Protect Different Stream Types? 

Protections for waters are under the Clean Water Act (CWA). This act states protection for Waters of the U.S. (WOTUS) but does not explicitly state the definition of WOTUS. The interpretation of the defined WOTUS varies across various presidential administrations, and legislative work towards a final definition began as of 2021. Currently, the CWA protects perennial and temporary streams.

These streams are protected due to their impact on water quality and general support of the environment. Depending on their pollutant and sediment supply, these waters can have devastating impacts on water quality. Additionally, this sediment transport can lead to fluvial landforms that alter the surrounding streambank ecosystem and riverine features and disrupt the natural distribution of these waters.

How to Identify Non-Permanent Streams? 

The professionals who identify non-permanent streams on project sites are environmental consultants. They are trained to discover environmental red flags on a site, such as these features, and ensure developers take proper precautions. The consultant will perform wetland delineation and stream identification during the environmental due diligence process.  The consultant will observe the following to identify streams:

• Hydrologic processes (ex. Evaporation rate)

• Geomorphology/physical processes (ex. Channel and bank presence and definition)

• Biological processes (ex. Flora and fauna present)

Non-perennial streams may see a lower or absent entrenchment ratio, the width of the flood-prone area to the bankfull width ratio, compared to perennial streams. This is due to the difference in flow regime experienced by these different types of streams. The presence of riparian vegetation and an absent or low gradient are some key features of these waters.

A stream reach assessment may be performed to determine if a stream can provide and supply clean water. Samples for stream reaches are taken at the most downstream point, where the latitudinal and longitudinal markers are. In a high gradient streams, these samples are taken in riffles and runs.

Cities and States record and map streams identified by consultants. These maps use the “blue line” method to chart these streams. Aspects of blue line maps are:

• Solid lines: indicating a perennial stream

• Broken lines: indicating an intermittent stream

• Contour patterns and hydrology knowledge are used to infer the location of ephemeral streams from the map

However, flaws occur within blue line maps due to the lack of extensive data regarding these non-permanent streams. 

The USGS exists as the sole scientific branch of the Department of the Interior and collects, analyzes, and maps environmental data relevant to U.S. trends. Organizations like the United States Geological Survey (USGS) use Geospatial Information Systems (GIS) and stream order data to create maps. However, these maps are subject to a similar lack of data as those mentioned in the methods above. 

As seen with GIS work, various software is being developed to automate this identification and mapping process. In one method, spatial data is captured via remote sensing technology to predict the likelihood of these streams’ presence in various areas. This technology uses vegetation presence as a tool to locate these waters. 

Transect uses machine learning to predict the instance of these waters. This automation uses past precipitation trends and the current streams known to create these predictions.