The River Continuum Concept: a model to explain the distribution of aquatic species

by Bert Cushing

Dr. Cushing is a professional stream ecologist with over 40 years of experience. He is an avid fly fisherman, lives in Estes Park, and serves on the BTWF Board of Directors. This article is adapted from an article originally published in Trout magazine, Spring 1995.

Flowing waters seem to have a special attraction to people, whether they are casual observers or actively interested. Yet most people probably do not fully appreciate the complexity of these ecosystems. The angler thinks mainly in terms of water, insects, and fish, but may not understand how these factors are influenced by, and related to, a host of other environmental factors. Similarly, stream restorers installing a log wing dam to create a pool understand the value of providing diversity of habitat, recognizing that pools offer resting sites for trout. But how does this pool change insect diversity, algal production, or organic matter processing and transport?

Keep in mind the fact that a stream is merely a reflection of the watershed in which it occurs. We cannot understand the ecological natures of the stream without integrating the ecological processes of its basin – the two are intimately intertwined.

Several physical and chemical characteristics operate to influence what is found at any place in a stream, and the most important of these are described below. These factors interact to shape the habitat, govern the richness, and eventually determine the biotic communities at each point in the continuum.

The South Platte River meandering through the plains of

 eastern Colorado. Photo from the USGS Ground Water

 Atlas of the United States, Chapter C by S. G. Robson

 and E. R. Banta, 1995.

Geology – The geological characteristics of the basin of a particular stream influence several aspects of the stream’s characteristics. First of all, it shapes the general channel characteristics of the stream. Rivers flowing through steep canyons in bedrock formations are largely restricted to these channels, and changes in the channel pattern occur extremely slowly. An example would be the Colorado River in the Grand Canyon. By contrast, such streams as the South Platte River in Colorado and Nebraska have soft sandy bottoms with wide flood plains along the edges. These streams meander back and forth, constantly changing the channel in which they flow.

The geology of a basin also contributes nutrients through the dissolution of the rocks. Streams in basins where the rocks have a low solubility are usually low in the essential growth nutrients nitrogen and phosphorus, compared to limestone streams whose rocks are readily soluble. Thus, the latter streams generally are higher in dissolved nutrients, enhancing the growth of algae in the stream and producing a larger food base for insects and other fish food organisms. This results in greater numbers and variety of insect species.

The geological structure of the stream basin also influences the interaction of the stream with another important physical variable, light. Light, of course, is important in photosynthesis, which is the basis of essentially all growth in terrestrial or aquatic ecosystems. Rivers or streams in steep canyons are obviously shaded for various portions of the day, thus decreasing the amount of light reaching the stream bed and slowing down the growth of algae during these times.

Light – Light directly influences the rate of photosynthesis of stream-dwelling plants such as algae, mosses, and rooted plants (macrophytes). The amount of light reaching a stream is influenced by the geological configuration of the basin, the shading by riparian vegetation, the amount of silt or other suspended matter carried in the water, the time of year, and weather conditions.

Current velocity – The current velocity is key to shaping the habitat and is governed largely by the gradient of the stream bed and the hardness of the substratum. Current velocities alternate as the water flows from riffles into pools, and in each we find different organisms adapted to these changed velocities. This is why organisms living in fast currents have flattened and streamlined profiles while those in pools are more robust.

The current is, of course, one physical variable that has influenced the evolution of organisms found in rivers and streams. Essentially, organisms have evolved to make use of the current in a positive way: they let the current bring them food rather than expending energy searching. The same current that brings food to organisms also transports waste products downstream. This principle, in fact, is what governs the entire structure of not only organisms but also stream communities along a river continuum. The waste products from the community inhabiting one reach of the stream become the food for downstream organisms, which have evolved to make maximum use of this input.

Temperature – Water temperature is a critical physical factor important for structuring river communities through its influence on such things as life histories, growth rates, metabolic processes, etc. Water temperatures generally increase downstream, the reflection of a longer period of exposure to warming by the sun. Man has a significant impact upon this process through use of flowing waters as receptacles of heated waste products and the use of the water itself to cool industrial processes. Increased water temperatures increase the rate of the metabolic processes such as growth, hatching time, and photosynthesis, but this is only true up to a point. When temperatures reach a critical point, they switch from being beneficial to being detrimental, eventually reaching critical levels that can result in the death of the organism.

Ambient temperatures largely govern the types of organisms found in a particular reach. In headwater reaches of a stream, temperatures are generally lower, and daily and seasonal fluctuations occur within narrower ranges. Here we find organisms adapted to these narrow ranges (called stenothermal organisms) such as trout, many mayflies and stoneflies, and grayling. Further downstream, thermal conditions have a much wider range of daily and seasonal fluctuations and organisms adapted to these conditions are called eurythermal; examples are carp, many minnow species, and mollusks.

Essentially, the physical and chemical factors provide the setting in which the biological communities exist. They require energy to live, grow, and reproduce.

Energy sources – Basically, food within a river or stream is produced by the same process that produces all of our food – photosynthesis by plants to produce organic matter, or primary production. Secondary production is the organic matter produced by the feeding of animals on this primary production. Sunlight to grass to cows on land is analogous to sunlight to algae to insects in a stream. Some fish, such as grazing suckers, feed directly on the algal food base, but trout don’t. They need intermediate consumers such as insects.

There are three main in-stream primary producers: algae, mosses, and macrophytes. The latter are the visible rooted plants such as pondweeds, water lilies, and watercress.

The most important of these in terms of overall energy for the ecosystem are the algae. Thus, where you find a situation in which sufficient light reaches the stream bed, the substratum is stable (i.e. rocks and rubble rather than shifting sands) and sufficient dissolved nutrients are available, a rich proliferation of filamentous green algae and/or microscopic diatoms will be present. The former are obvious to the naked eye and appear as green, hair-like tufts, sometimes attaining lengths of a meter or so. Common genera are Cladophora, Ulothrix, and Spirogyra. These are often mistakenly referred to as “moss.” We will discuss moss below. The most widely proliferating of the various algae are the diatoms, the microscopic, silica-coated cells which form the brown, slippery coating on virtually every solid underwater object. Truly, they are the “grasses of the waters,” just as are the grasses of our pastures and the phytoplankton (floating algae, including diatoms) of the oceans. They are responsible for most of the primary production occurring in rivers and streams and hence, one of the two most important food resources for higher organisms.

The second in-stream primary producers are the true mosses. Despite the wide use of their name, they are infrequent in most of the waters we fish. True mosses are small, leaf-bearing plants and are largely restricted to the colder, headwater reaches of streams. They can be abundant, coating both rocks and wood – such as tree trunks that have fallen into the stream bed. Because of their restricted distribution, mosses are not important as an overall energy resource in entire stream systems, but can be locally important in headwater reaches. They not only produce oxygen and food for herbivores, but also provide shelter for organisms living in the stream.

The third of the in-stream primary producers are the macrophytes, the large rooted plants. Where the current slows and deposition of sediments provides a suitable substratum for plants to take root, we can find large stands of submerged and emergent plants. These are commonly observed in some of the most popular fishing streams such as in the headwaters of the Madison River in Yellowstone Park and the famous spring creeks in Montana. Interestingly enough, despite the large amounts of plant tissue which are present, these plants do not provide a significant food resource for other stream organisms, at least not while they are growing. Only after they die and decompose, breaking the tissue down into finer particles, do these plants become a rich source of food. While living, though , macrophytes provide living surface and protection for a wide variety of small aquatic organisms.

Until we recognized the importance of the terrestrial environment to stream ecosystems, stream ecologists concentrated their attention on the primary producers within the stream, but we now recognize the importance of primary production originating outside of the stream. Primary production occurs in the terrestrial environment in the form of leaves, sticks, cones, or fruits which fall into or are transported into the stream, or even entire trees which end up within the river – anything organic which originated from outside the stream or river.

Collectively, this material is called Coarse Particulate Organic Matter (abbreviated CPOM) and it is identified as any particle of organic matter greater than 1 millimeter in size; dead and decomposing macrophytes are also part of this CPOM pool as long as they meet this size criterion. What happens to this material to convert it from a largely unpalatable mass of cellulose, lignin, and other organics into a palatable food source for in-stream organisms? First, immersion in the water leaches various soluble chemical from the tissues; then it is rapidly colonized by a wide variety of aquatic bacteria and fungi, which use the remaining plant tissues as a source of energy to produce more bacteria and fungi – a rich source of protein for stream organisms. A colleague of mine likens it to peanut-butter on crackers; we tolerate the crackers to get the peanut-butter. For the CPOM, the insects ingest a certain amount of plant tissue to get the bacteria and fungi. We call this process of bacterial and fungal breakdown “conditioning.”

What is happening to these energy resources, the in-stream primary producers and the CPOM from the terrestrial environment, during their existence in the stream or river? The in-stream producers may be eaten or eventually die and decompose, drifting downstream, while the terrestrial CPOM is slowly converted to finer particles through ingestion and defecation by organisms and physical abrasion by the current. In any event, the CPOM is gradually and eventually converted to finer and finer particles, and we call this Fine Particulate Organic Matter (FPOM, less than 1 millimeter in diameter). This huge pool of microscopic organic matter and the living diatoms form the main energy resources for stream organisms.

So far I have described the physical and chemical conditions that are important in providing the basic “template” of our stream ecosystem, and have explained the sources of energy which provide the nutrition for the higher levels of the food-web in our stream continuum.

Among the members of the aquatic community that use the primary production of the plants, the insects are usually dominant and provide the best examples of the organism inhabiting the various niches in a stream or river.

The traditional way of grouping aquatic insects for discussion is by order, e.g., Trichoptera (caddisflies), Ephemeroptera (mayflies), Plecoptera (stoneflies), Diptera (true flies, midges), etc. This organization is valuable and instructive, but for a full appreciation of their ecological role in stream ecosystems we must look at how the insects function as users and transformers of energy.

Here again, tradition usually presents this information in terms of what the insects eat. However, stream ecologists have recently turned to another form of classification of stream insects in this respect, one which can more accurately describe their function and niche in stream food webs. This classification system is based not on what the insects eat, but how they obtain their food. These categories are called functional groups, and as the name implies, denote their functional role when describing how and where they feed.

Basically there are four functional groups:

Shredders – These are the insects that feed directly on CPOM, ingesting large pieces of organic matter and converting them to FPOM through maceration, defecation, and physical degradation. Common shredders include the stonefly Pteronarcys californica, the famous salmonfly of the Madison River; cranefly (Tipulidae) larvae; and larger instars of several caddisflies in the Family Limnephilidae.

Grazers – Sometimes called “scrapers,” these insects have mouth parts adapted to feeding on the layer of diatoms and other algae growing on the surface of rocks (collectively, this film is called periphyton). Common grazers are the caddisflies Glossosoma and Dicosmoecus (the October Caddis) and many mayflies such as the genus Stenonema and some baetids.

Collectors – As their name implies, collectors simply collect food particles, FPOM mainly, from a variety of locations.
There are two groups of collectors; the “filtering-collectors” who filter FPOM from the water column in a variety of ways, and the “gathering-collectors” who gather detrital particles from various locations on the stream bottom.

Larva of Hydropsyche, in its silken net for filter feeding.

Image courtesy of Uta Gruenert.

Filter Feeders – One group of filtering collectors builds fixed retreats with an upstream opening across which they spin a fine silken net. After the net has collected sufficient food, the insect will either selectively graze the particles from the net, or may devour the net, food and all, and then spin a new net. Examples of these are the ubiquitous caddisflies of the family Hydropsychidae, probably the most widely distributed group of caddisflies in streams worldwide. Other filter feeders use specialized body features. Brachycentrus, the small caddisfly that builds “log cabin” cases, cements one edge of the case to a stone or stick, and extends its six legs, each heavily fringed with fine hairs, into the current. When sufficient FPOM has been collected, it retracts its food-laden legs and uses its mouthparts to remove the food. Mollusks and other fine-sediment inhabiting invertebrates, such as chironomids, filter organisms and food particles from the water by pumping water through their bodies or through tubes in the mud and extract the food particles.

Brachycentrus larva.

Photo by Dean Hansen.

Gathering-Collectors – Probably the most dominant functional group in terms of diversity, these insects collect deposited FPOM from under and between rocks, in the interstitial spaces below the stream bed – wherever it occurs. Some of the genera most familiar to fly anglers are found in this group and include such genera as Tricorythodes, Baetis, Ephemerella, and others.

Larva of Baetis harrisoni. Photo by Ron Griffith.

Predators – As the name implies, these are the insects which feed on other organisms and are essentially the same as the carnivores. Most beetle larvae are predators, as are most of the stoneflies and one family of caddisflies, the Rhyacophilidae. One group of stoneflies, however, are mainly shredder.

Each species or even group of species may not remain in the same functional group throughout its life in the stream. Early larval forms of some Limnephilidae perform as grazers following hatching, scraping periphyton from rocks. However, as they mature and grow, they require a more robust food source and become shredders, utilizing the larger CPOM sources of food. Also, many perform the role of more than one functional group, although one role is usually dominant. For example, filtering Hydropsychidae may also be predators if their nets filter micro-crustaceans.

The River Continuum Concept

This concept explains how all these factors interact to produce the changing mosaic of insects from headwaters to river mouth. In the headwaters, the stream is narrow and generally well-shaded by the riparian canopy. Thus, insufficient light reaches the stream bed to promote algal growth and the current is too fast, substratum often unsuitable, and nutrients too low to promote growth of macrophytes; mosses are the dominant in-stream primary producers. In headwater streams, the amount of primary production (measured as amount of dissolved oxygen produced by the in-stream primary producers, plants and algae) is less than the respiration (use of dissolved oxygen by aquatic animals). Since the stream is not producing enough energy to supply the demands of the stream organisms, energy from outside the stream is necessary in the form of CPOM from the terrestrial environment. Heavy riparian growth and small surface area mean a lot of woody debris enters the stream, so shredders and collectors are numerous here. Grazers are essentially absent because of the dearth of algae, and predators are few.

In the stream’s mid-reaches, the stream bed has widened, the bottom is well-lit, temperatures have warmed, and nutrients have accumulated, leading to a proliferation of algae on the bottom. Rooted plants will occur in protected places where sediments have accumulated. At the same time, CPOM input has decreased on an area basis because of the wider stream. In-stream primary production in this reach would exceed respiration, and the excess production is transported downstream.

Here the collectors are about as numerous as they were in the headwaters, although they are typically different species, but the major change is in the reversal of the shredders and grazers. CPOM is low and algae prevalent, so shredders are rare and grazers are as numerous as the collectors. Predators make up about the same proportion.

By contrast, in the large, usually slow-flowing and deeper lower reaches of a river, the increased turbidity of the water prevents sunlight from supporting algal growth on the bottom. In-stream primary production now takes place within the water column where suspended algae (phytoplankton) flourish, and macrophytes are abundant in the shallow backwaters. Terrestrial input of CPOM is insignificant. The water column contains large amounts of suspended FPOM from the excess production of the mid reaches, and respiration of this material exceeds the primary production of the algae and macrophytes. The benthic community is largely made up of collectors, both filterers and gatherers. Shredders and grazers are absent. Most of the collectors present are sediment dwellers which construct burrows or tubes in the mud and scavenge or filter the suspended FPOM. Again, predators are present in about the same relative numbers, but are different species.

The River Continuum Concept outlined here has been tested in streams around the world and, for the most part, its principles have been supported. Problems occur, however, when comparing its predictions in streams that have unique characteristics or stream where changes occur from man-made alterations in the pristine pattern.

Several mechanisms, such as riparian removal, logging, and damming interrupt the pristine conditions in a stream continuum. When conditions are changed, there is usually a shift in the predicted patterns. For instance, when a small tributary typical of a headwater reach enters a larger, mid-size or large stream, the insect community immediately below the confluence contains functional groups characteristic of the tributary, rather than the receiving stream. We call these reset mechanisms. This is because the conditions in the receiving stream have been “reset” to those higher in the continuum. The same thing happens when the riparian canopy in headwater reaches is removed; more sunlight reaches the stream, periphyton algae occur, and grazer organisms are found. Thus, the stream pattern has been “reset” to the patterns expected in the mid-reaches. This condition also occurs in western streams that have less dense riparian canopies in their headwaters than those of eastern deciduous forests where the River Continuum Concept model was formulated. Damming drastically resets the downstream continuum pattern, and the changes are related to where on the continuum the dam is located, whether it has a surface (warm) or bottom (cold) release of water, and several other factors.

Streams function as dynamic, holistic systems, systems which are an integrated reflection of all of the components both within and outside of the stream. Streams are not just individual segments, but are longitudinally linked, with each section dependent upon and shaped by upstream reaches. I hope that whether or not you try to remember all of the details I’ve discussed, that at least you can have a fuller appreciation of the stream environment.