Streams are some of the most important freshwater bodies on virtually all types of land masses. They are responsible for irrigating wetlands, transporting crucial nutrients over considerable distances, washing away impurities, and being a source of hydration for wildlife. They are sites of reproduction and maturation for many economically important species. Moreover, their consistent flow can serve as a renewable source of hydropower.
Given the sheer diversity of our landscapes and the global variation of climate conditions, streams can take on many forms. These life-giving bodies of water may be temporary or permanent, torrential or slow-moving, incredibly shallow or deep, and narrow or immensely wide. They may meander, split, or converge into smaller or larger bodies of water, shaping terrestrial environments as they evolve.
Whatever their shape, length, or permanence, streams are liquid corridors through which the highest mountains may ultimately be connected to our oceans. Due to their importance, it has been necessary to develop classification systems. The rationale behind the determination of these systems, as well as their specific categories of streams, are discussed below.
Why Is It Necessary to Classify Streams?
Stream classification may serve as a tool with which to determine conservation strategies, identify differences and similarities between waterbodies, and make inferences about their behavior. Classifying streams also aids in properly naming them, keeping researchers up to date on their trends, and tracking their natural evolution. Over space and time, a small stream can turn into a gushing river. To describe them as “fluid” would be apt in many ways.
The tricky part, as a researcher, student of freshwater biology, or legislator, would be choosing exactly which classification system to adopt. The criteria of these vary as some place more weight on the permanence of streams, whereas others may highlight their depth, width, or connectivity with other bodies of water.
One of the simplest classification systems makes use of the concept of “stream order”, which organizes streams based on their relative size. For example, as the stream order increases (from the 1st-order stream to the 12th-order stream), their overall volume may increase. A 12th-order stream would thus be a vast river.
Another classification system is based on stream permanence over a single year. Other systems focus on their morphological pattern, direction of flow, and their tendency to branch off or rejoin other streams. At times, ecologists adopt a multi-leveled classification system of streams. This may initially focus on stream morphology and then further classify streams based on their specific features. To simplify matters, this article will touch on the major types of streams from the most direct classification systems.
Types of Streams Based on Stream Order
1) 1st-order stream
The smallest type of stream, this waterway has no other streams flowing into it. It is considered a headwater stream because it is found in the upper reaches of a watershed system. It usually forms on steep slopes and quickly flows downward, eventually converging with another 1st-order stream to form a new 2nd-order stream. A 1st-order stream is often referred to as a tributary.
2) 2nd- and 3rd-order streams
The 2nd-order stream is the flowing body of water below which two 1st-order streams converge. This likewise occurs on steep slopes and flows downward into another convergence zone, from which a 3rd-order stream arises. The 3rd-order stream is the largest of the headwater streams. Most of the world’s waterways are classified as these first-order types of fast-moving streams.
3) 4th- to 6th-order streams
These are medium-sized streams that carry sediments, debris, and runoff from the headwater streams. These have an increasingly larger volume, as every convergence leads to an increase in the physical dimensions of the waterbody. Unlike headwater streams, however, these medium-sized streams tend to flow more slowly and are less steep.
4) 7th- to 12th-order streams
These large streams are considered rivers. They carry notably large volumes of debris, runoff, sediments, and nutrients from both headwater streams and medium-sized streams. As these are usually slower-flowing, more types of plants and animals can live in them. Some examples of these streams include the Amazon River, which is the only 12th-order stream, and the Mississippi River, which is a 10th-order stream. 7th- to 12th-order streams are more likely to be permanent, though their water levels fluctuate throughout the year.
Types of Streams Based on Permanence
1) Perennial streams
As these streams are present all year round given normal rainfall levels, they may also be referred to as ‘permanent streams’. Their water levels may fluctuate, but they always have flowing water present in a portion of their stream bed. These water bodies are usually found in downstream areas, into which the baseflow of smaller streams converge. The consistent water flow may prevent the development of roots, so these types of streams are unlikely to be extensively covered in vegetation.
Key characteristics of perennial streams include well-defined channel banks, riffles and pools, marks of water fluctuation, wetland vegetation, connectivity with seeps or springs, signs of debris movement, algae-covered sediments, and aquatic fauna (e.g. benthic macroinvertebrates, small fish, insect larvae).
2) Intermittent streams
These types of streams have a baseflow at least once a year. Water is usually present at some parts of the stream bed in winter and spring. During these seasons, the groundwater level tends to be higher than the bed of the stream channel, increasing the chances of water flow. A discontinuous flow is more likely to be observed in summer and fall.
The physical and hydrological features of intermittent streams are less well-defined than those of perennial streams. Key characteristics of these waterways include indicators of fluctuating water levels, a well-defined channel, minor signs of debris movement, temporarily present leaf litter, the presence of hydrophytic vegetation, and a lower diversity (compared to perennial streams) of aquatic insects.
3) Ephemeral streams
Ephemeral streams are named for their short-lived and transient nature. They are present only during and shortly after rainfall events that induce a considerable flow of water. The duration of their flow is not long enough to create the physical and biological characteristics of both intermittent and perennial streams. Regardless, they remain to be ecologically important as they recharge groundwater systems and support the survival of wildlife.
As ephemeral streams are located above the groundwater table, they rely wholly on stormflow as a source of water. Thus, they remain shallow and are unable to form a well-defined stream channel. The sediments of ephemeral streams are poorly sorted. Since water is only briefly present in the stream bed, the fibrous roots of many plants may develop.
Special Classifications
These notable types of streams are categorized according to their morphology or their tendency to separate and reconverge. Streams are so diverse and are affected by both spatial and temporal factors throughout the year, so there are dozens of highly specific classifications. Those listed below are the most readily recognizable types.
1) Alluvial streams
Alluvial streams are technically “distributaries” because they branch out from existing streams and do not return to the main stream system. These types of waters are created by streams gushing forth from a steep area and abruptly flowing into a relatively flat plot of land. The resulting arrangement is the division of the source stream into many smaller ones, usually arranged in a fan-like or “E”-shaped orientation.
These types of streams accumulate a large number of sediments, largely due to the eroded canyons through which they flow downstream. The sediments gradually settle over a distance. Often, these streams end with the deposited sediments themselves. As the rate of stream flow can change over time, the distribution of sediments may not be uniform.
2) Braided streams
A portion of a braided stream may initially resemble the appearance of an alluvial fan. Technically, a braided stream is a single waterway that becomes split into smaller streams. If you follow the journey of these separate streams, you’ll find that they reconverge to essentially form the same stream once more. As suggested by the word “braided”, they separate only to rejoin.
Unlike the triangular buildups of sediments created by alluvial streams, those found between braided streams have a bar-like appearance. These streams tend to occur in narrow valleys that are devoid of actual floodplains.
3) Meandering streams
Have you ever looked out of a plane window and spotted a glistening, snake-like body of water winding through an expanse of flat land? That was most likely a meandering stream. This type of stream, which is normally associated with vast plains, obtains its form through both erosion and deposition.
As water quickly moves along a curved streambank, it erodes the sediments along the outer bend of the bank. The sediments are subsequently deposited along an inner bend downstream. Over time, deep pools form along the outer bends of the streambank as more sediments are removed. The waving pattern created by the water’s velocity around the bends is fairly ubiquitous, and it can be quite difficult to come across streams that maintain a straight morphology in flat landscapes.
4) Straight channel system
Though streams are rarely ever straight, there are a few exceptions. These may be present where an existing fracture, such as a fault, in the bedrock may influence the path through which the streambed develops. The path may cause the stream to have a seemingly uniform velocity on both sides – if not for the entirety of its length, then simply for a significant portion of it. Any alterations to the velocity on either side of the stream can change its direction and deposition patterns.
Researchers specializing in straight channels have found that their waters still move in a sinuous or winding fashion, however. This is because the deepest part in one section of the channel can have a higher velocity. Beneath what appears to be a uniform surface flow of water is actually an alternating arrangement of pools and sediment bars.
Young vs Old Streams
Over time, streams go through many phases of development. Terrestrial alterations, anthropogenic activity, and climate change can significantly alter their appearance and ecological importance. At its initial phase of development, a young stream may start off as a narrow and shallow body of water. Events like landslides or dam construction can eventually alter its pattern and flow, causing it to either dry out or form new connections with other bodies of water. The laws of physics will also force it to evolve.
Naturally, heavy erosion drastically changes how a stream looks and moves over both time and space. Thus, mature streams may be somewhat unrecognizable compared to their initial forms. Young streams or rivers may seem to follow a straight path, creating a V-shaped valley in the landscape. Mature streams, which can start to curve or meander, are more associated with wider, U-shaped valleys.
The oldest streams have wide curves and are associated with flat landscapes. The velocity with which their waters flow is much slower than that of young streams. These changes aren’t applicable to all types of streams, of course. Each one matures in a different way and can evolve into other types of streams. This emphasizes just how dynamic freshwater systems can be!
