Complex | CSKT | Division of Fish, Wildlife, Recreation, & Conservation

The Four C's: Complex

What Causes Streams to Become Complex?

Undisturbed streams are almost always complex because natural stream processes are intact and functioning normally. Among the most important processes and factors responsible for creating complex habitat are:
Click on the tabs below to learn more about the parts of a complex stream:

Natural flows, including periodic large floods that move materials on the stream bed and rearrange point bars
The stream having access to its floodplain and the freedom to migrate across the floodplain
A complex and healthy riparian community
Lots of large woody debris in the stream
The presence of beavers, and
The presence of groundwater upwelling zones

Click on the tabs below to learn more about the parts of a complex stream:

What Causes Streams to Become Complex?

Undisturbed streams are almost always complex because natural stream processes are intact and functioning normally. Among the most important processes and factors responsible for creating complex habitat are:
Click on the tabs below to learn more about the parts of a complex stream:

Natural flows, including periodic large floods that move materials on the stream bed and rearrange point bars
The stream having access to its floodplain and the freedom to migrate across the floodplain
A complex and healthy riparian community
Lots of large woody debris in the stream
The presence of beavers, and
The presence of groundwater upwelling zones

Click on the tabs below to learn more about the parts of a complex stream:

Pools

Complex stream habitats include frequent pools—roughly 20 per stream mile. In pools, the water slows, eddies, and can even back eddie (flow upstream) or stop. As the deepest and often the slowest parts of any given reach, pools provide holding and resting habitat for adult trout. They serve as refugia (areas that an organism can use to survive a period of unfavorable conditions) during low flows or hot days.

Pools are also used as feeding areas, especially for big trout. The fish will cruise or patrol in a particular pattern when the feeding period comes. And because stream banks are often undercut in pool areas, pools tend to be places where large woody debris (logs) get added to the stream. Large woody debris greatly improves habitat for fish and insects. Aquatic insects that are often found in pools include dragonfly larvae, scuds, aquatic worms, damselfly larvae, cranefly larvae, and leeches.

Finally, it is important to know that there are different kinds of pools: meander-formed pools, log-enhanced or rootwad pools, boulder-formed pools, and bedrock-formed pools. All are critical for the survival and successful reproduction of migratory salmonids like bull trout. In short: if you want good trout habitat, make sure you have pools.

Glides

A glide, located just below a pool, is the transition zone between pool and riffle. It is where the bed of the stream is rising up to meet the start of a riffle. At the same time the surface of the water is dropping (so the slope of the water surface is opposite that of the stream bottom).

In a glide, the water moves slowly and has little turbulence—glides are called glides because the water surface is smooth. The stream bottom—made up of fine gravel, sand, and organic matter—is intermediate between that of pools and riffles.

Glides, with their quiet water, are important places for trout to hold or feed in (studies show bull trout often select glides and pools over riffles).

Riffles

Riffles are the short, straight parts of a stream midway between two meanders. In riffles, the water is shallow. It is rushing and bubbling over and through gravels, cobbles, and boulders, its surface agitated with many small waves. The water, because it is turbulent, is highly oxygenated, which is one of the main reasons riffles are valuable as habitat.

Not only do riffles aerate the water in a stream—that is, put dissolved oxygen into the water—they also help purify the water by expelling carbon dioxide from gravels (the gas is produced by the breakdown of plant and animal waste). Indeed, trout spawning habitat is typically found at the head of riffles because trout eggs need clean, oxygen-rich water flowing over and through gravels.

Riffles also support the highest aquatic insect biomass, density, and diversity in a stream. Common insects found in riffles include caddisflies, mayfies, blackflies, water pennies, and some stoneflies.

Aquatic insects are, of course, the most important food source for fish. Most sizes of trout can live in riffles because they can hold in calm spaces next to the bottom and behind larger rocks and boulders. Juvenile trout often feed at the downstream end of riffles because of the abundance of food. So like pools, riffles are critical stream habitat.

Runs

Runs have moderate gradients, moderate flow velocities and depths, a variety of substrates (bottom materials). Their flow is somewhat turbulent (they have small waves) but the water surface is unbroken. In short, runs are deeper but not as fast as or turbulent as riffles. Riffles are most often followed by runs.

Runs support moderate to high aquatic insect biomass and are important feeding habitats for trout. As flows drop during the summer, runs become even more important (because riffles may become too shallow). Studies in Canada have shown resident populations of bull trout prefer run and pool habitat in natal streams with cobble and gravel bottoms.

Quiet Water

Quiet-water areas are particularly critical to the survival of trout during their first year. They include backwaters, small spring-fed channels along stream margins, floodplain ponds and sloughs, and alcoves (alcoves are protected pools with very slow water generally found along the sides of the stream but within the active channel). These rearing areas provide low velocities, a steady supply of small food particles, shade, and refuge from larger predatory fishes, birds, and mammals. As rearing areas these off-channel and in-channel habitat features are a vitally important part of the habitat mix.

Undercut Banks

Undercut banks provide cover for trout. Cover is critical for the survival and successful reproduction of migratory salmonids like bull trout. Spawning trout can be highly visible and vulnerable to terrestrial and avian predators (including humans).

Undercut banks provide shade, which moderates water temperatures and gives trout a refuge from hot summer temperatures, especially during low flows. Undercut banks can also provide physical shelter from high-flow events, which can be quite important.

Recent research has found that undercut banks are also important to aquatic insects. One study in a Wyoming subalpine-meadow stream showed that in July mayflies, stoneflies, caddisflies, beetles, aquatic flies, and water mites were all substantially more numerous in samples from undercut banks than from riffles or pools. During cooler months undercut banks were not as important to insects.

Spring-fed Channels

In natural streams, spring-fed side-channel habitat is constantly created and abandoned as the river migrates laterally and changes course. It is usually associated with former stream channels abandoned through natural process. Spring-fed side channels derive a major portion of their flow from either groundwater or seepage from the adjacent stream.

Spring-flowing channels and backwater wetlands—are valuable habitats that benefit a variety of aquatic and terrestrial species. They provide critical habitats for both juvenile trout and a variety of wildlife species, including amphibians, reptiles, birds, mammals, and mollusks.

Spring-fed channels can also function as spawning, rearing, and overwintering habitat for fish as well as providing a refuge from floods. Under most circumstances, a side channel provides more stable flow and temperature regimes than the main channel. Like quiet-water areas, spring-fed channels provide low velocities, a steady supply of food, shade, and refuge from larger predatory fish.

....

Woody Debris

Large, downed trees and other large woody debris in the channel and on floodplain surfaces are not only key habitat elements, they also help develop valuable aquatic habitat. Large woody debris acts to deflect flows, which creates slower areas in the stream where large fish often rest. Often large woody debris deflects the flow so that it scours out deep pools. It can trap sediments and fine organic material that contributes to the aquatic food web. Log jams cause temporary obstructions to the stream course that, during peak flows, promote local channel switching and floodplain flooding, helping to rejuvinate backwater, slough, and springbrook habitats. In other words, large woody debris provides a diverse and stable habitat mosaic critical to many aquatic organisms, including trout.

Log jams also force surface flows into alluvial aquifers, promoting the exchange between surface and ground water, which is beneficial to fish and insects.

Large woody debris can also act to divert and break up ice accummulations in winter. This helps prevent the formation of ice drives that can channelize rivers.

Beaver Ponds

Beaver dams and the foraging activities of beavers create ponds that buffer flows of water and the downstream movement of organic matter, nutrients, and sediment. Beaver ponds are important rearing and wintering areas for trout. They also promote the movement of stream channels across the floodplain and the complexity of channels, thereby maximizing the structure of trout habitats and increasing the exchange of surface and subsurface waters, which also benefits fish.

Another under-appreciated function of beavers may be their unique role of transporting willow and other important riparian species upstream. Many of these branches sprout and grow into bushes. Thus beavers help willows and other riparian plants recolonize areas after landslides, severe drought, and other catastrophes that can wipe out riparian plant communities in tributaries.

Groundwater Upwellings

Deep pools, low-velocity backwaters, and springbrooks isolated from main channel flows are common zones of upwelling. Groundwater upwellings create diverse thermal refugia for fishes and other organisms. These habitats are cold relative to warm surface waters in summer, warmer than surface waters in winter, and can sometimes be nutrient-rich and highly productive.

In winter, groundwater-influenced stream habitats often remain free of anchor and surface ice, buffering them from the stresses of winter freezing and thawing processes that can be highly disruptive for aquatic organisms. In summer, groundwater-upwelling areas are known to be used heavily by trout that inhabit warmer river reaches.

Groundwater-upwelling areas are well known to provide important spawning habitats for fall-spawning bull trout. Groundwater-rich pools also provide critical winter habitat for juvenile and adult trout, which may move long distances to congregate in these areas.

Upwelling areas on alluvial reaches are hot spots of production because nutrients accumulate in groundwater flow areas. Those nutrients greatly stimulate primary production and likely increase protein content of some aquatic plants and riparian woody vegetation. Hence, riverine habitats influenced by ground water provide a more consistent and abundant food supply for all life stages of trout and aquatic insects.

Complex interactions between ground water and surface water are key attributes of high-quality riverine habitat for salmonid fishes.

Click to expand

Pools

Complex stream habitats include frequent pools—roughly 20 per stream mile. In pools, the water slows, eddies, and can even back eddie (flow upstream) or stop. As the deepest and often the slowest parts of any given reach, pools provide holding and resting habitat for adult trout. They serve as refugia (areas that an organism can use to survive a period of unfavorable conditions) during low flows or hot days.

Pools are also used as feeding areas, especially for big trout. The fish will cruise or patrol in a particular pattern when the feeding period comes. And because stream banks are often undercut in pool areas, pools tend to be places where large woody debris (logs) get added to the stream. Large woody debris greatly improves habitat for fish and insects. Aquatic insects that are often found in pools include dragonfly larvae, scuds, aquatic worms, damselfly larvae, cranefly larvae, and leeches.

Finally, it is important to know that there are different kinds of pools: meander-formed pools, log-enhanced or rootwad pools, boulder-formed pools, and bedrock-formed pools. All are critical for the survival and successful reproduction of migratory salmonids like bull trout. In short: if you want good trout habitat, make sure you have pools.

Glides

A glide, located just below a pool, is the transition zone between pool and riffle. It is where the bed of the stream is rising up to meet the start of a riffle. At the same time the surface of the water is dropping (so the slope of the water surface is opposite that of the stream bottom).

In a glide, the water moves slowly and has little turbulence—glides are called glides because the water surface is smooth. The stream bottom—made up of fine gravel, sand, and organic matter—is intermediate between that of pools and riffles.

Glides, with their quiet water, are important places for trout to hold or feed in (studies show bull trout often select glides and pools over riffles).

Riffles

Riffles are the short, straight parts of a stream midway between two meanders. In riffles, the water is shallow. It is rushing and bubbling over and through gravels, cobbles, and boulders, its surface agitated with many small waves. The water, because it is turbulent, is highly oxygenated, which is one of the main reasons riffles are valuable as habitat.

Not only do riffles aerate the water in a stream—that is, put dissolved oxygen into the water—they also help purify the water by expelling carbon dioxide from gravels (the gas is produced by the breakdown of plant and animal waste). Indeed, trout spawning habitat is typically found at the head of riffles because trout eggs need clean, oxygen-rich water flowing over and through gravels.

Riffles also support the highest aquatic insect biomass, density, and diversity in a stream. Common insects found in riffles include caddisflies, mayfies, blackflies, water pennies, and some stoneflies.

Aquatic insects are, of course, the most important food source for fish. Most sizes of trout can live in riffles because they can hold in calm spaces next to the bottom and behind larger rocks and boulders. Juvenile trout often feed at the downstream end of riffles because of the abundance of food. So like pools, riffles are critical stream habitat.

Runs

Runs have moderate gradients, moderate flow velocities and depths, a variety of substrates (bottom materials). Their flow is somewhat turbulent (they have small waves) but the water surface is unbroken. In short, runs are deeper but not as fast as or turbulent as riffles. Riffles are most often followed by runs.

Runs support moderate to high aquatic insect biomass and are important feeding habitats for trout. As flows drop during the summer, runs become even more important (because riffles may become too shallow). Studies in Canada have shown resident populations of bull trout prefer run and pool habitat in natal streams with cobble and gravel bottoms.

Quiet Water

Quiet-water areas are particularly critical to the survival of trout during their first year. They include backwaters, small spring-fed channels along stream margins, floodplain ponds and sloughs, and alcoves (alcoves are protected pools with very slow water generally found along the sides of the stream but within the active channel). These rearing areas provide low velocities, a steady supply of small food particles, shade, and refuge from larger predatory fishes, birds, and mammals. As rearing areas these off-channel and in-channel habitat features are a vitally important part of the habitat mix.

Undercut Banks

Undercut banks provide cover for trout. Cover is critical for the survival and successful reproduction of migratory salmonids like bull trout. Spawning trout can be highly visible and vulnerable to terrestrial and avian predators (including humans).

Undercut banks provide shade, which moderates water temperatures and gives trout a refuge from hot summer temperatures, especially during low flows. Undercut banks can also provide physical shelter from high-flow events, which can be quite important.

Recent research has found that undercut banks are also important to aquatic insects. One study in a Wyoming subalpine-meadow stream showed that in July mayflies, stoneflies, caddisflies, beetles, aquatic flies, and water mites were all substantially more numerous in samples from undercut banks than from riffles or pools. During cooler months undercut banks were not as important to insects.

Spring-fed Channels

In natural streams, spring-fed side-channel habitat is constantly created and abandoned as the river migrates laterally and changes course. It is usually associated with former stream channels abandoned through natural process. Spring-fed side channels derive a major portion of their flow from either groundwater or seepage from the adjacent stream.

Spring-flowing channels and backwater wetlands—are valuable habitats that benefit a variety of aquatic and terrestrial species. They provide critical habitats for both juvenile trout and a variety of wildlife species, including amphibians, reptiles, birds, mammals, and mollusks.

Spring-fed channels can also function as spawning, rearing, and overwintering habitat for fish as well as providing a refuge from floods. Under most circumstances, a side channel provides more stable flow and temperature regimes than the main channel. Like quiet-water areas, spring-fed channels provide low velocities, a steady supply of food, shade, and refuge from larger predatory fish.

....

Woody Debris

Large, downed trees and other large woody debris in the channel and on floodplain surfaces are not only key habitat elements, they also help develop valuable aquatic habitat. Large woody debris acts to deflect flows, which creates slower areas in the stream where large fish often rest. Often large woody debris deflects the flow so that it scours out deep pools. It can trap sediments and fine organic material that contributes to the aquatic food web. Log jams cause temporary obstructions to the stream course that, during peak flows, promote local channel switching and floodplain flooding, helping to rejuvinate backwater, slough, and springbrook habitats. In other words, large woody debris provides a diverse and stable habitat mosaic critical to many aquatic organisms, including trout.

Log jams also force surface flows into alluvial aquifers, promoting the exchange between surface and ground water, which is beneficial to fish and insects.

Large woody debris can also act to divert and break up ice accummulations in winter. This helps prevent the formation of ice drives that can channelize rivers.

Beaver Ponds

Beaver dams and the foraging activities of beavers create ponds that buffer flows of water and the downstream movement of organic matter, nutrients, and sediment. Beaver ponds are important rearing and wintering areas for trout. They also promote the movement of stream channels across the floodplain and the complexity of channels, thereby maximizing the structure of trout habitats and increasing the exchange of surface and subsurface waters, which also benefits fish.

Another under-appreciated function of beavers may be their unique role of transporting willow and other important riparian species upstream. Many of these branches sprout and grow into bushes. Thus beavers help willows and other riparian plants recolonize areas after landslides, severe drought, and other catastrophes that can wipe out riparian plant communities in tributaries.

Groundwater Upwellings

Deep pools, low-velocity backwaters, and springbrooks isolated from main channel flows are common zones of upwelling. Groundwater upwellings create diverse thermal refugia for fishes and other organisms. These habitats are cold relative to warm surface waters in summer, warmer than surface waters in winter, and can sometimes be nutrient-rich and highly productive.

In winter, groundwater-influenced stream habitats often remain free of anchor and surface ice, buffering them from the stresses of winter freezing and thawing processes that can be highly disruptive for aquatic organisms. In summer, groundwater-upwelling areas are known to be used heavily by trout that inhabit warmer river reaches.

Groundwater-upwelling areas are well known to provide important spawning habitats for fall-spawning bull trout. Groundwater-rich pools also provide critical winter habitat for juvenile and adult trout, which may move long distances to congregate in these areas.

Upwelling areas on alluvial reaches are hot spots of production because nutrients accumulate in groundwater flow areas. Those nutrients greatly stimulate primary production and likely increase protein content of some aquatic plants and riparian woody vegetation. Hence, riverine habitats influenced by ground water provide a more consistent and abundant food supply for all life stages of trout and aquatic insects.

Complex interactions between ground water and surface water are key attributes of high-quality riverine habitat for salmonid fishes.

Click to expand