This is a question that could launch a debate even between seasoned hydrologists and freshwater biologists. “River” and “stream” are words that are often used interchangeably to refer to a body of water that flows in one direction. Off the bat, most people would base their usage of either word on the size and perhaps the importance of the freshwater feature in question. Technically, however, both words could infer the same image and meaning.
The same goes for the difference between a stream and a creek, brook, or rivulet. These are associated with the ribbons of water flowing from watershed regions in mountains. If the flow of water is strengthened or stifled by natural factors, altering their physical dimensions, would one term then take the place of another? Brooks can turn into streams and streams might evolve into forceful rivers. Their fluid (pun intended) nature is precisely what makes them quite complicated to differentiate!
Hydrologists thus prefer to use “stream” as an umbrella term for practically all water bodies that flow downstream. Instead of simply using a fixed width, depth, or length to set the criterion for naming these natural features, they consider their permanence, various spatial scales, and their connectivity with other freshwater features. Thus, a river is actually the largest type of stream, but there are many properties that set it apart from its smaller counterparts.
What Exactly Is a River?
While it may be acceptable to narrow down the differences between a stream and a river to their width and depth, this could unfortunately discount major aspects of their habitat structure and diversity. Essentially, as “seventh-order streams”, rivers are the convergence zones of both small and large streams. A bird’s eye or aerial view of a river system would reveal a tree-like structure, where the river appears to be the trunk and the streams resemble branches.
As they flow out into the sea, rivers gain more water and enlarge in width. Their forceful nature, which is enough to lift substrates and erode the shoreline, results in an open canopy that sheds light onto typically turbid layers of water. The combination of a steady current and fine substrates leaves many organic particles in suspension, so the water column serves as a vital food base for aquatic organisms.
Simply put, a river is a large, flowing body of freshwater that usually exits into the sea or ocean, with water sourced from several streams. It has diverse habitats with features that are varied due to unique structures along the river’s source, banks, and mouth. During flood events and droughts, the physical features of a river may change to erode or reveal islands, channels, and gravel bars.
Ecological Differences Between Streams and Rivers
Unlike rivers, smaller streams (first to third-order streams) don’t have mouths that exit into the oceans and seas. Instead, they are more closely linked to wetlands in higher altitudes. They serve as the dynamic bridges that close the gap between rivers and terrestrial sources of water (e.g. springs, aquifers, precipitation, etc.). Larger streams may exit into rivers or lakes, depositing organic particles and nutrients.
“Streams” tend to be smaller and shallower than rivers, may have overhead tree canopies that interlock to produce shade, and are often situated along a more severe slope. Thus, their waters are usually cooler, less turbid, more highly oxygenated, and more abundantly supplied with leaf litter and general detritus. Collectively, these natural aspects influence the diversity of microhabitats and the types of fauna they support.
As streams enlarge but remain shallow, resulting in increased light penetration due to an open canopy, their waters become warmer. With sunlight and warmth, submerged flora can thrive in the benthic zone. Moreover, plankton communities may become more diverse and complex, sustaining the needs of many small aquatic faunas. An overall increase in primary productivity is usually observed in sunlit streams. These are able to retain a highly oxygenated profile due to the turbulence in their riffle zones.
Habitat Diversity and Fish Zonation in Streams and Rivers
Along a single continuum of unidirectional water flow, there can be several types of streams. The smallest streams gradually develop into larger, complex ones with more points of convergence with other water bodies. As they flow downward, their light penetration, depth, gradient profile, substrate composition, width, oxygen levels, and water velocity inevitably become altered by changes in the landscape and climate.
Collectively, these abiotic properties shape the habitats in streams. Each is associated with a unique set of physical conditions and microhabitats which sustain different types of biological communities. As a result, shallow streams with less light, cooler temperatures, and larger substrates can support a community that is totally different from that of fully exposed and low-gradient rivers.
- Steep, high-gradient streams located at higher altitudes are generally shallow and sheltered under the shade of canopies. As they collect leaf debris, their biological communities may largely be composed of animals that shred leaves and consume particulate matter along the benthos. These steep streams may form pools with small fish and macroinvertebrates.
- At lower altitudes, moderate-gradient streams with an increased depth and width tend to have more grazers and complex communities of macroinvertebrates. Grazers may dominate due to the light-induced increase in periphyton production. The vegetation, in turn, opens up many microhabitats and sites for shelter and egg-laying. An increase in the average size of fish species may be observed.
- Low-gradient rivers may be dominated by “collectors”, which consume suspended fine particles. As these rivers are wide enough to be well-exposed to sunlight and have reduced tree canopy coverage, lower diversity of leaf-eating organisms may be present. In the deepest rivers, the size range of fish species is significantly increased and many apex predators may be present.
Permanence: Streams vs Rivers
All kinds of streams, including rivers, are dynamic freshwater systems with properties that regularly shift and change throughout the year. These changes are more pronounced in regions experiencing seasonal changes. In tropical to subtropical regions, periods of prolonged rainfall and dryness can drastically alter the physical parameters of streams and rivers. However, due to their sheer size, depth, and access to multiple sources of water, rivers are generally more permanent than streams.
Small streams that tend to cut into the terrestrial landscape, creating a permanent and continuous depression, may dry out completely. These are called intermittent or seasonal streams. Those that are largely supplied by groundwater sources may be present all year round. These are referred to as “perennial streams”. Essentially, a river would fall into this category.
Major rivers normally see a consistent flow of water through a full cycle of seasons, even if many of its associated streams dry out. Most of them are permanent despite lengthy dry spells, but their water levels may significantly drop. Some, such as the Colorado River, may fail to reach the coastline due to prolonged droughts and water removal in key points of the river.
The Journey From Gully to River
Without its many headwater streams and tributaries, rivers may fail to gain enough strength to form and reach their coastal or lacustrine destinations. Likewise, streams rely on smaller ribbons or gushes of water to merge, allowing them to take shape and form a connection between the primary water source and the ocean. The largest river system, that of the Amazon, is fed by thousands of smaller streams and is the result of fluvial processes caused by their convergence.
Creeks, brooks, and rivulets are smaller streams that initially form due to the pull of gravity on high-altitude waters. The sloping terrain of mountains allows groundwater, rainwater, or melting ice to travel downhill in the form of gullies or rills. The smallest and most impermanent of streams, these gullies are so narrow and shallow that animals can simply hop over them without getting wet. Once they dry up, they may not leave any impressions on the landscape.
When gullies from different sources join to form a steady flow of water, a small stream is finally formed. Small streams continue to flow downward due to the sheer pull of gravity. As they do so, they continue to be innervated by runoff, groundwater, and other streams until they are large enough to become the headwaters of major river systems. Thus, the resulting rivers carry a rich mixture of substrate deposits, organic materials, and nutrients from diverse landscapes and various water sources.
The Importance of Rivers and Streams
Knowing the similarities and differences between rivers and streams is key to understanding just how complex and vital they are. Collectively, they are the arteries that bring nourishment to wetlands, allowing for the growth and survival of flora and fauna. Their reliable connectivity, consistent flow, and structural diversity are precisely what makes them so important as sources of nutrients, food, and energy.
The deliberate conversion of one part of a river system leads to major consequences. Exposing one part of a stream to pollutants can alter the ecology of its downstream river. Damming a section of a headwater stream can also change the population structure of the river it originally led to. Ultimately, anthropogenic disturbances to streams and rivers can affect coastal and oceanic life.
