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 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.
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 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.
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. Eventually, if these bends increase to be too large, the stream will connect these loops as means to a more efficient flow. This 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 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.