Streams (channelized, flowing freshwater also known as lotic systems) account for 0.001 % of Earth’s water. Yet they are extremely important to human societies and house a great deal of the planet’s biodiversity.
Terms such as river, creek, and run often confer an idea of the size of a stream, but are used arbitrarily. “Stream” is the generic term or catchall word for any part of a network of channelized flow.
Food chains tend to start with land plants growing along stream banks, including overhanging trees. Organic nutrients enter the stream as dead leaves and fallen branches or stems and trunks. This detritus is then broken down by a variety of detritus feeders and consumed by invertebrates, especially insect larvae living in and on the channel bottom. Decomposers such as bacteria are also important in releasing nutrients into the stream ecosystem. The invertebrates of the channel bottom are food for a variety of bottom-dwelling fishes that are consumed by predatory fishes that in turn become meals for fish-eating birds and mammals.
Detritus is collected throughout the watershed and moved downstream, necessitating the continual input of new material.
Freshwater enters a stream or drainage system as run-off, percolating soil moisture, or slowly flowing groundwater. It carries dissolved nutrients and other minerals as determined by the bedrock geology. Water chemistry is thus directly influenced by such inputs.
Streams and climate
The regional climate affects streams especially by determining amounts and seasonality of precipitation. The volume of water entering the stream, particularly from runoff, in combination with the nature of the substrate, determines the amount and size of sediment particles that can be carried and whether scouring and erosion or deposition will occur on the stream bottom. The composition of bottom materials and the depth of the channel are always changing in response to the flow regime.
Streams can be categorized according to the time period each year that their channels hold flowing water. Three basic types occur:
- Permanent streams flow year-round. A special type is the exotic stream, which flows through arid lands and collects water only in headwater regions under more humid climates.
- Intermittent streams have water moving in their channels only during or for a few weeks or months after the rainy season. They are usually associated with climate types with strong seasonal patterns of rainfall, such as tropical wet and dry climates (savanna climates), semiarid and arid climate types. However, in humid climates in temperate zones, spring snowmelt may result small intermittent streams in forests and woodlands.
- Ephemeral streams are those that contain water only during and immediately after a rainstorm. Common in desert climates, they can be associated with flash floods resulting from distant heavy rains well upstream.
Streams are pretty much linear habitats in which the small streams of the upper portions of the river system influence what happens in the larger waterways of the lower reaches. Scientists conceptualize and organize the network of streams connected in a single drainage system by talking about stream order. The pattern begins with first-order streams, the initial trickle of channelized flow that collects at a river’s source. Higher order streams are recognized when two streams of the same order join. For example, two first-order streams will merge to form a second-order stream; two second-order streams join to form a third-order stream, and so on. Among the world’s largest rivers, the Nile and Mississippi rivers are tenth-order streams, the Amazon a twelfth-order stream.
Any lower order stream that enters a larger, higher order stream is deemed a tributary of that stream. First-order streams have no tributaries, but collect and channelize water from runoff or springs.
Each stream segment drains a definable bowl-shaped area of land known as its watershed. Watersheds of lower order streams are nested or embedded in the greater watershed of the mainstem river, which has its outlet at the mouth of the entire river system. Rivers and their watersheds develop together through processes of erosion, sediment transport, and deposition.
River Continuum Concept
Stream life and ecosystem processes vary rather predictably with stream order, as headwater streams (orders 1-3) grade into medium streams (orders 3-6); and those grade into large, higher order (>6) streams. Changes in environmental conditions such as particle sizes on the stream bed, degree of shading, water temperature, and inputs of terrestrial detritus influence the composition of the invertebrate fauna and, to a somewhat lesser degree, the fish fauna. Fishes in headwater streams tend to be cold-water species adapted to fast-flowing water, while warm-water species dominate large, lowland reaches of the river system. Fish populations in headwater streams are isolated by conditions downstream and thus there tend to be more endemic species there than in larger order streams. Headwaters are also more vulnerable to stream piracy by the backward erosion of fast-moving streams, a process that allows for dispersal of species from one drainage system to another.
Especially in the lower reaches of large drainage systems, cyclical flooding of the floodplain is a significant factor in stream ecology. The Flood-Pulse Concept notes the link between a river and its floodplain, especially in major tropical systems. The annual flood can greatly expand the area underwater, creating seasonally flooded forests and/or wetlands and floodplain lakes such as exemplified in Brazil’s Amazon rainforest and Pantanal. River fishes may synchronize their reproductive cycles to coincide with new areas and flood sources made available by flood conditions.
Life in streams
One way to classify life in streams is by where it occurs. Those organisms living on the water’s surface, mostly insects, are known as the pleuston. Water striders and other members of the group rely on the high surface tension of water to support them. Other organisms live as a film on rocks are members of the periphyton. This group is composed of bacteria, fungi, and algae trapped with small particles of detritus and silt in a gelatinous slime. Algae that cling to the stems and leaves macrophytes are categorized as epiphytes. Most non-fish animals are confined to the benthos, the community that lives on the channel bottom. These species are largely invertebrates, including mollusks, crustaceans, and especially the abundant larvae of diverse insects.
Plants. Mosses and liverworts along with red algae (Protista) occur on the downstream side of rocks in fast-flowing streams of the upper reaches of a stream system. Rooted emergent and submerged plants are more typical of lower reaches where shallow, slower-moving water provides suitable habitat near the banks. In the tropics it is common to find large mats of floating plants such as water hyacinth (Eichhornia crassipes).
Animals. Invertebrates, especially insect larvae, dominate the benthos, as mentioned above. Some invertebrates feed on coarse organic particles, shredding it to eat the tissue between the veins of dead leaves. Others feed on rotting wood and/or perhaps the microorganisms that are decomposing it. Filter-feeders such as blackfly larvae depend upon the fine organic particles created by detritus-feeders.
Among vertebrates. fish are of course most characteristic. Different taxonomic families dominate different continents. Birds and mammals are often closely associated with streams, few are actually residents of the water. Dippers and a few ducks come close to being full-time dwellers in the water; as do beaver, river otter, capybara, hippopotamus, and duck-billed platypus. True stream mammals include close relatives of marine mammals such as the river dolphins of South America, South Asia, and China as well as the Amazon manatee.
Adaptations of animals to the stream environment are generally mechanisms to prevent their being swept downstream. Physical aspects of their bodies that work to do this include:
- the streamlined body shape of many fishes
- small, rounded bodies with flat ventral sides of bottom-dwelling fish in fast upland streams. These fish may also have spiny pectoral fins to help them wedge into crevices or between rocks and mouths turned downward to scrape food from channel sediments.
- flattened bodies of many invertebrates that let them crawl under stones or debris on the stream bottom
- suckers or suction pads on feet as seen in leeches and snails or the friction pads of small, movable spines found on water-penny beetles that let them cling to stones
- hooks, grapples, and claw-like legs that let insect larvae attach to stones in the stream channel
Caddisfly larvae display a behavioral adaptation that has them cement heavy sand to their bodies to create ballast to prevent them from drifting up into the current.
Many attached invertebrates let go every now and then and float short distances downstream to take advantage of fresh food supplies or to avoid predators.
Note: Each river system is unique in terms of its physical and biotic conditions. A good source of information on the world’s major river systems is Rivers Network: http://www.riversnetwork.org/rbo/index.php.
To see good photos of freshwater microorganisms, visit http://nwnature.net/micro_org/index.htm. At the same site are also good photos freshwater macroinvertebrates such as insect larvae, worms, and mollusks (http://nwnature.net/macros/index.html). These photos are copyrighted, so do not use without permission.