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Basics of Stream Ecology

      No ecosystem can be studied in isolation. If we were to describe ourselves, our histories, and what made us the way we are, we could not leave the world around us out of our description! So it is with rivers: they are directly tied in with the world around them. They take their chemistry from the rocks and dirt beneath them as well as for a great distance around them.

The Watershed

      The area that a stream drains is called its drainage basin, or catchment area. We also often call it the river's watershed. Water flows downhill (unless it evaporates, freezes, or is taken up by plants). So when it falls as rain, melts from snow or ice, or bubbles up from beneath the earth, it will either flow one direction or another, and always downhill. If you were to stand by a stream at the bottom of a valley, you might look up and see hills all around you. All the land you see that slopes downward toward your river is part of the river's drainage basin. Some drainage basins are so unbelievably huge that you can't even see where their downhill flows begin. For instance, if you stood by the Mississippi River, you couldn't see the Rocky Mountains. Still, much of the water that falls as rain or snow in the Rocky Mountains ends up in the Mississippi River.

What the River Carries

      First, of course, streams carry water. We're so familiar with water that we don't often stop to think about what it really is. Once you are ready to leave this website, you might want to stop by the links page to learn about one of our planet's most amazing substances.

      When water rolls down the slopes of a watershed (or drainage basin or catchment area), it carries things with it. It dissolves chemicals and carries them. It carries particles of dirt. If it is meltwater from a glacier, it will carry glacial flour, which is sediment the the glacier has made by grinding the rock beneath it very finely, making the water look almost milky. And it carries organic matter: tiny bits of leaves, bacteria, and a lot of other things too small to see. As it flows, it grows to rivulets, and carries larger bits of matter. By the time the water gets all the way down to the river, it is full of whatever was on (and in) the land around it. The river can carry sticks, leaves, logs, brush, and even sand, pebbles, rocks, and boulders.

      There are other ways that things can end up in a river. Winds can blow in sediment (particles of dirt) and bits of organic matter. A lot of living things like insects depend on flowing water to carry out their life cycles. Birds leave urine, droppings, and feathers. Other animals visit the river and often leave their waste in it. Many animals die in the river, adding their organic materials to the water.

      Natural events can occur that alter the river's ecology by changing these kinds of factors. Mudslides, heavy rainfall, and fires can make drastic changes. When Mt. St. Helens erupted in Washington State, it sent uncounted tons of fine ash in a torrent down through its river valleys. These events (though they seem extreme from our human point of view), are a very large and slow, but nevertheless integral, part of the river's ecosystem. While Mt. St. Helens's ash was destructive to the salmon that used its rivers, the mountain has erupted many times in the long history of the salmon, and they have adapted in their own mysterious ways. We are far from understanding everything about biological adaptation.

Even Gases Get Involved

      Rivers are closely tied with the atmosphere (the air above them). Gases from the air, like oxygen, carbon dioxide, and nitrogen dissolve into the water. The colder the water is, or the more it churns as it flows downhill, the more gases there will be in it. Rain scrubs molecules and other particles out of the air, and when the air is polluted by coal-burning power plants and automobile exhaust, the rain becomes acidic, and falls as acid rain. Acid rain can lead to the death of streams and lakes. There are dead lakes New England as well as in Sweden, caused primarily by burning coal for energy. Water that is too acidic falls on the earth and leaches minerals out of it, washing them into streams and lakes. Some of these minerals, like aluminum, are highly toxic to fish. Aluminum in water causes fishes' gills to become covered with thick mucus so that they are no longer able to breathe. Fish in acidic lakes simply suffocate to death.

The Riparian Corridor

      The narrow area alongside a stream that has its own special vegetation is called the riparian corridor or zone. What plants you will find in a riparian corridor depend on where the river is: the continent, the climate, stream hydrology, geology, alkalinity of the soil, and many other factors. Riparian zones contribute nutrients, shade, organic materials for small organisms to eat, soil stability, and habitat. They also contribute food for fish in the form of bugs dropping from branches.


     Every stream also carries life-forms and the habitats in which they live. Plants, diatoms, fungi, larvae, crustaceans, mollusks, worms, fishes, mammals, and many other life-forms live in and utilize streams.

     Diversity is key to the survival of a stream's life-forms. Genetic diversity must be present within each species. Species and biological diversity must be present as well--many different types of animals. None of these types of diversity can be created or fostered by humans. They are the products of millions of years of evolution by trial-and-error. Once they are lost, they cannot be re-fashioned by scientists. Finally, habitat diversity is essential. Each stream contains many different habitats and microhabitats. A single species may require several different habitats to carry out its life functions, and each habitat is inhabited by its own species that cannot live elsewhere.

     Keystone species are those species whose functions are so intertwined with the lives of other animals in a system that their disappearance will cause the system to become imbalanced or even collapse. The beaver is a perfect example of a keystone species. It dams rivers, creating ponds and wetlands that support an entire system of stream organisms. When beavers are removed from a stream, many of those stream organisms are displaced or die.

     To explore habitats, foodwebs, and saving endangered fishes, click on one of the books in the graphic above.


      Floods are natural events, but their influence on river ecology is subtle. Animal and plant communities in rivers have spent millions of years adapting to the conditions around them, and floods have become simply a part of a larger cycle of river ecology for them. Riparian corridors depend almost exclusively upon their streams' flooding cycles for their existence. Many fishes wait until the first sign that the annual spring flood has begun to start breeding. Many insect larvae wait for flooding to begin to lay eggs, hatch, or metamorphose. Flooding provides a bonanza in cheap new food sources for stream denizens. Floods flush insects, bugs, and worms that used to be on land into the stream, which become dinner for fishes. Flooding results in increased fertility for the river. The more fertile a river, the more invertebrates will be able to live in it--and invertebrates form the base of the foodchain. Nutrients (like nitrogen and phosphorus) are washed out of soil and animal feces. Nutrients added to the shallow, warmer waters of the floodplain lead to extra growth of plankton. Floods also wash dead brush and trees into the stream, providing habitat for countless animals.

      If you guessed that people can have an effect on rivers, you were right. People, in fact, have more effect on rivers and streams than any "natural" events possibly could. The more people use a river or live near it, the stronger their effects on it become.


      Some of the ways we change rivers are familiar. For instance, nearly all the rivers in the United States have been dammed. People dam rivers in order to store water for drinking, washing, farming, watering lawns, swimming pools, agriculture, and countless other uses. They also dam them to control floods downstream, and in order to make electricity.

      Damming rivers changes their ecology forever. Each stream has its own biological community, all members interacting with each other in a complex fashion, all depending on each other for their survival. Dams (by changing flows, temperatures, and water clarity) change those communities. Habitats are removed. Slight changes in a stream's insect populations end up affecting the top of the foodweb, like kingfishers, bald eagles, osprey, herons, grebes, and bears.

     Additionally, salmon, many trout, bass, shad, and some minnows migrate up rivers in order to reproduce; if they can't get all the way up a river, or if their offspring can't get all the way back down, reproduction fails. Soon, there are fewer and fewer of them, and ultimately they will disappear forever. Many species of salmon have recently been listed as endangered species, primarily for this reason.

      Of course, many dams have fish ladders--but fish ladders are not always successful. And on the trip back down to the sea, many juvenile fish die in the turbines of the dam or after falling the long distance from the reservoir to the river below the dam. It isn't the fall that hurts them; rather, they get "the bends," like any diver, from high concentrations of nitrogen gas in the turbulent waters below the outfall. They also experience increased mortality in the large, quiet reservoirs behind the dam.

      In the Southwest, effects of dams are dramatic. There, damming is often done for two purposes: flood control and control of water resources. We have already seen how flood control can be detrimental to a river. Control of water resources in the Southwest is now out of control: so much water is needed for people to use every day, as well as for irrigation of crops, that many of the rivers run dry at least once a year.

     The Rio Grande has been allowed to run dry every spring or summer for many years, and as a consequence, the Rio Grande silvery minnow is now an endangered species. The Santa Fe River is almost always dry. The Colorado River, most of which is diverted to California, Las Vegas, and Arizona, does not even reach the sea anymore. It is being used to water large cities, pools, lawns, and golf courses in the desert, and to irrigate agricultural products that never could have existed in these arid lands naturally, such as central California's rice paddies. Water is piped in from desert rivers to the paddies, and from there it evaporates into the hot, dry air.

      To learn more about dams, visit Ecology of Dams.

Channelizing Streams

      When a stream is prone to flooding, or to meandering out of control and across property lines or roads, we often channelize it. We may dry up whole sections of stream in order to bulldoze it into a tidy, straight line of water. We may try to protect ourselves from its unruly behavior by lining the stream's banks with concrete or riprap (small boulders crated by truckloads and dumped along the sides of water channels). This kind of channelization leads to loss of both stream and riparian habitat. It also increases the destructive potential of the river.

      A channelized stream becomes poor in nutrients and habitat. Without periodic flooding, its riparian zone is starved of water and nutrients. Stream inhabitants depend on the riparian zone for food, shade, and debris. Channelization creates artificial river banks without variation, while stream inhabitants depend on natural variations such as backwaters, riffles, embayments, and large woody debris for shade or warmth, cover, protection, and food. This means that channelized streams will no longer support the kind of fish that people would like to see in their rivers, though they may support some fishes like carp that are not so welcomed by people. The townspeople lose, too, because ironically, the more you try to channelize a river, the more out of control it becomes. Erosion, a minor irritant before, threatens property, buildings, and roads.

      Flooding becomes more catastrophic when streams are channelized. Water gathers energy as it flows downhill. When a stream meanders, it creates banks. The water then 1) pushes against the banks, and 2) swirls in eddies. In both cases, the energy of the flowing water is decreased. When a stream is channelized, however, there is nothing to prevent it from gathering more and more destructive energy as it flows downhill. Secondly, a healthy floodplain acts as a sponge, soaking up floodwaters, while channelized rivers simply forward the extra water downstream until it overwhelms dams, dikes, or walls. Finally, when rivers are channelized, people are encouraged to live on floodplains, risking lives and property in the event of a catastrophic flood. The inevitable response to catastrophic flooding is, unfortunately, to increase channelization--which leads to even more catastrophic flooding.

     Channelization also causes other undesirable changes. River deltas such as the Mississippi River Delta are constantly sinking, but their sinking is counter-balanced by the river's adding more silt. As a result, the delta appears to remain stable. When people channelize deltas (by building canals and lining streams) to control flooding so they can build houses and cities, as was done in New Orleans, no more silt is deposited. The result is that the delta simply sinks--and the city, now below sea level, becomes prone to catastrophic flooding.

     Channelization also cause streams to sink. In its natural state, a stream winds back and forth across the landscape, much like a snake. It cuts new channels but also lays down new silt as its velocity slows. The result is that what was a canyon eventually becomes a valley with a level, fertile valley floor: the floor is made of the stream's silt deposited over millions of years. When the valley fills up with houses, people want to channelize the stream so that it won't wind back and forth anymore, cutting across their property lines or flooding their homes. Confined to its channel, the sole activity of the stream becomes cutting--and so it cuts downward and banks are replaced by steep walls. It will continue cutting downward until it reaches bedrock. This has happened to the Santa Fe River within the city of Santa Fe, New Mexico.


      We also change our rivers by changing the land around them. If we pave land or remove vegetation from it, rainwater runs directly off of it instead of soaking into the earth. This urban runoff carries pollutants like car oil and pesticides instead of nutrients. When we change the vegetation around a stream, we change its chemistry. For instance, a developer may cut down all the trees around a stream in order to place a big neighborhood of houses next to it. This has many effects, among them that no more leaves will fall into the stream, taking out the very base of the stream's foodweb. Tree branches will no longer shade the stream and it will become too warm for the fish that belong there, and choked with algae. In addition, without overhanging branches, bugs will no longer fall from them to feed fish. The trees themselves were critical to that stream because they were providing nutrients to it, as well as shade for the growth of other important streamside vegetation. And finally, without the roots of vegetation to anchor streamside soil, the soil will become eroded away by the stream--forcing homeowners to channelize the stream.


      As wooded areas become popular places to live, logging increases in order to build more homes. Logging in itself is not always such a harmful thing to streams: it is the logging roads that must be built for the logging trucks that do the lion's share of the damage. Silt from these dirt roads washes down the hillsides with the rain and enters the river, choking the substrate by filling in the spaces between gravels and cobbles of the streambed. This eliminates an important habitat of many of the aquatic insects that fish eat. Without habitat, the insects disappear. It also makes the maturation of salmon and trout impossible. Salmonid eggs (and later the very small juvenile salmon, or alevins) spend their early lives buried in streambed gravels, sheltered from the river's current and hidden from predators. They live off their yolks until they are large enough to fend for themselves, before emerging into the water column. While they are still in the gravels, water must flow rapidly over them to bring them fresh, dissolved oxygen and to carry their wastes away. When silt from development fills in the spaces between the rocks, salmon and trout can no longer grow there.

      In some rivers in America, especially those that run through clay soils, silt from logging--or, more often, from farming and development--can cloud up the river, blocking light. When light is blocked from a river, a whole different set of plants and animals grows and the original community is lost. Carp, fishes that are not native to America but have populated many of its streams and lakes, will also silt up rivers as they root about in the mud looking for food.

Urban Runoff

      After land is logged (unless the loggers own the land), people start to move in, filling up the watershed with buildings and pavement. This means that there will be more oil, and more lawn fertilizer, herbicides, and pesticides. It also means pollutants will increasingly run straight into streams. Because each house must have a sewage system to dispose of its inhabitants' wastes, many of these sewage systems will eventually leak their contents into the drainage basin and then into the river. Where city sewer is provided, treated effluent will be discharged directly into the river. Pollutants from this urban runoff, leakage, and disposal include chemicals that fertilize the river, changing its ecological balance, chemicals that kill bugs and algae that form the bottom of the foodchain, and chemicals that build up in animal tissues to later poison humans and predators. While most people think of stream pollution as consisting of mercury, PCB's, and other industrial pollutants, our streams are increasingly contaminated by the drugs we take, from caffeine and painkillers to sex hormones and the many pharmaceutical products we take to control our mood, cholesterol, and other modern problems. These drugs are not filtered out by most water treatment plants.

Disappearing Streams

      Much of the U. S. Southwest's water is drawn from wells. Because of population growth, so much water is being drawn out that the level of the aquifers (water tables) are dropping. The ground water doesn't have a chance to recharge. People are having to drill deeper and deeper to find water. As the levels of aquifers drop, springs and seeps dry up--and so do streams.

      A number of Southwest's cities--including Santa Fe and Albuquerque--have been faced with the prospect of having to restrict growth due to disappearing aquifers. Instead, they build pipes beneath major rivers like the Rio Grande to remove water from beneath their riverbeds. Curiously, many people believe that this water is separate from the river water and that this kind of extraction will not draw down the rivers any further. This is wrong. It is in fact the same as removing water from the rivers themselves. The politicians who, in order to support increased development, have pushed for this measure may recognize that drawing water from beneath a river is an effective way to thwart Western water law. People (including Texas and Mexico) downstream of these cities will, as a result, find far less water available to them. To learn more about hydrology and/or Western water law, see How a River Flows or visit the Reading List.

Mining and Minerals

     With population increases come soaring demands for minerals, from the minerals that are used to generate electricity, manufacture cement for construction, and build appliances and electronic devices, to the gold in the jewelry that people want to wear. The extraction of these minerals comes with a heavy cost to the public. Nearly all mining requires the use of a great deal of water, lowering aquifer levels and drying up streams, wetlands, and lakes. Much mining activity leads to the leaching of acids into groundwater and streams, and toxic heavy metals as well as radioactive substances are becoming common mining pollutants. See the Pollution page to learn more about this problem.

Invasive Species

      Perhaps the greatest effect that we humans have had on streams is the introduction of fish--and sometimes other organisms--that don't belong there (invasive species). Some of this has been accidental, like the appearance of animals from peoples' bait buckets or aquariums, or the appearance of the species like the zebra mussel from ship's holds. Much of it has been intentional. We raise the kinds of fish that people want to catch when they go fishing in a river in hatcheries. Then we take them out in trucks and dump them in the river ("stock" the river) for the benefit of fishing enthusiasts. Most often these are trout and bass, which are predators that can quickly unbalance the ecology of a stream. Species that are introduced to a river may wipe out native species by eating them, displace the stream's native species by filling niches more successfully, or fill a niche that previously wasn't filled.

     Because trout and bass are "top predators"--are at the top of their food webs--they have a lot of control over the makeup of the community beneath them in the food chain, and alter the stream's ecology permanently. Because we don't usually stock the river with what was originally in the river (indigenous or native species), but rather with what fishermen like to catch the most, this often means that native fish will be driven out, or even interbreed with the newcomers.

     This is what is currently happening in the American West: cutthroat trout are interbreeding with stocked rainbow trout, and slowly the many strains of cutthroat are being erased. Rainbow and brown trout compete more effectively for food and for breeding opportunities than do most native trout. Yet they do not (because they are usually hatchery fish) reproduce effectively. Hatchery fish sometimes also introduce new pathogens to rivers (like whirling disease), causing illness and death of the resident fish. One way or another, many native trout will eventually succumb to the invasion of new species that were not a part of the original ecosystem.

      Much the same thing happens on a smaller scale with native minnows in Southwestern streams--only the villain this time is usually bait-bucket introductions rather than hatcheries. Some minnows--and crayfish--are bred for bait, used for fishing in pristine waters, and released at the end of the fishing day.

The Future

      We have not yet destroyed our rivers, although very few remain in their original condition. Disaster looms for many of them. The science of river ecology has advanced. We are learning to build logging roads in ways that minimize erosion, to locate mining operations more wisely, to control runoff from industry and agriculture, to maintain riparian (vegetated) zones around rivers, and to allow them to flow naturally. We are attempting to protect fish species that have been listed as endangered species.

      Unfortunately, scientists cannot do these things alone, and they daily battle huge corporate, bureaucratic, and political obstacles to environmental improvements. The United States's environmental agencies have been rendered almost completely ineffective due to relentless pressure from extremely powerful and wealthy persons, as well as from the politicians to whom they contribute money. In the end, only the citizens of a country, working together, will be able to protect their streams from ecological destruction.

      The biggest job remains with you and me. Human population growth continues, placing ever more pressure on our streams and rivers. It is up to each of us to take measures in our personal lives that will assist them in remaining healthy for our children to enjoy.

Play a Watershed Game! Be an Explorer or a Detective!

ball graphic Watch an amazing video describing a trophic cascade: "How Wolves Change Rivers."

ball graphic To find out more about ecology, return to the menu above to select a link.
ball graphic To learn how to monitor a stream or do a bioassessment, go to the links page.
ball graphic Learn how to save a stream or endangered species.
ball graphic To learn about animals or plants in a stream, go to Stream Residents and Visitors page and follow the menus.
Reading listCheck out the reading list, where you will find some excellent references listed. (You will be able to order them from here, too).

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The number of adaptations made by organisms to their surroundings in streams alone is so long that if one were to start listing them now, one would probably still be listing them long after retirement! Naturally, we can't cover all of them here--only a few. There are several basic ways for an animal--or a plant--to adapt.

Genotypic changes, such as mutations or recombinations of genes, tend to be great enough to separate closely related animals into species. An example might be a salmonid that has evolved an subterminal mouth (below the snout) in order to eat from the benthos. Phenotypic changes are the changes that an organism might make during its lifetime to better utilize its environment. An example might be a fish that changes sex from female to male because of an absence of males. Behavioral changes have little to do with body structure or type: a fish might spend more time under an overhang to hide from predators, for instance; or, lacking a mate of its own species, it may hybridize with another. Ontogenetic change is that which takes place as an organism grows and matures. An example would be a coho salmon that inhabits streams when young, and migrates to the sea when older, changing its body chemistry to allow it to tolerate saltwater.

Mouths are often the only obvious way some small fishes can be told apart; they change morphology depending on the food the fish eats. The arrangements of the jaw bones and even other head bones, the length and width of gill rakers, the number, shape, and location of teeth, and barbels all change to allow fishes to eat just about anything found in a stream. Killifishes' mouths are turned up to the surface, and they have flat heads to allow them to get close to the surface, because that's where they find their food. Suckers and sturgeons have inferior, often jawless mouths, to allow them to eat from the bottom. Predators often have highly evolved head and jaw structures that allow them to protrude their mouths, take big gulps, and suck in prey. They also often have many backwards-pointing teeth, and may even have teeth in their throat and on their tongue--all in order to keep a small fish from escaping their mouths. Shad have long, fine gill rakers, because they filter fine food from the water column. Catfish, bullheads, carp, and chubs all have barbels to "taste" with, in order to find their food on the bottom.

Shape changes to allow a fish to do different things in the water. Sculpins, chubs, catfish, and dace have body shapes and fin shapes that push them down in the water, against the substrate (bottom), and allow them to hold their place against even strong current. Predators such as pike, trouth, perch, bass, sunfish, and gar have evolved an arrangement and shape of fins that allows them to lurk without moving, then lunge suddenly to catch their prey. Salmonids have a body shape that allows them to hold a position facing upstream in a fast-moving current.

Color may change within hours, to camoflage, or within days, or may be genetically predetermined. Salmon are dark dorsally, and silvery ventrally, to hide them from predators both above and below. Parr marks on young may help to hide them in turbulent stream environments. Fish tend to turn dark in clear water, and pale in muddy water. Colors may also be used to signal readiness for mating.

Aestivation, lungs, and ability to hold onto oxygen helps fishes survive in arid desert climates, where lakes and streams may dry up from time to time. Aestivation refers to the ability of some fishes to burrow into the mud and wait out a dry period.

Schooling serves as protection for many fishes, particularly those that are subject to predation. On the other hand, some predators school as well--such as tuna and barracuda--as a way of making their hunting more efficient.

Buffering Acid Rain

The ability of a water body to buffer acid rain--to make it less acid--depends on what its chemistry was before the acid rain fell. The behavior of carbonates is what buffers the acid. A water body with certain carbonates present is termed, "alkaline." Carbon may dissolve directly into the water as CO2 (carbon dioxide), to attach to water molecules, becoming H2CO3 (carbonic acid). This is a weak acid (the acid in acid rain is a strong acid), and it dissociates--breaks apart in water--to form a balance between H2CO3 molecules, H+(hydrogen ions), HCO3- (bicarbonate ions), and CO32- (carbonate ions). HCO3- and CO32- react in turn to produce OH- (hydroxyl ions). All these negative ions are very effective at capturing the loose H+ ions that come with acid rain. The ions associate and dissociate in the water to form an equilibrium--at a fairly neutral pH. When a strong acid is added, like nitric or sulfuric acid from pollution--the reactions are forced in another direction, and the acid is neutralized. The ability of a lake or river to do this is called its buffering capacity.

The greatest source of alkalinity in water bodies is Ca2CO3 (calcium carbonate), which comes from limestone. It is especially common in the Southwestern U.S.. The Southwest deserts were at one time underwater, and the deposits of millions of years worth of seashells and foraminiferan shells led to the formation of limestone, which now leaches into the rivers and acts as a buffer.

Mountain areas, however, typically have a mainly volcanic history; there is little or no limestone, but plenty of igneous rock, which is acidic. This is one reason mountainous areas suffer most from acid rain. New England may not be mountainous, but it was at one time. Much of its soil still comes from igneous rock

     In the Pacific Northwest, riparian corridors tend to include willows and alder. Both of them like to have wet roots. Alder has the special ability to take nitrogen gas out of the air and make it into biologically useful nitrogen (nitrogen fixation). This makes the soil near streams and on floodplains more fertile. Cedar trees also like being near Northwest streams, but a little further away: damp soil is good for them, but not wet soil. Cedar trees also like the acidity of Pacific Northwest streams and watersheds.

      Willows can be found right next to Southwestern U.S. rivers, often in dense thickets. Cottonwood trees stay a little further back from the river, forming the outer part of the corridor. Like most trees in the poplar family they like water, but not too much. Unlike Pacific Northwest rivers, Southwestern rivers tend to be very alkaline--and cottonwoods like that. Most coniferous trees like piņon pine stay far enough away from the floodplain to avoid getting wet roots. There are other conifers, like alligator juniper, that will appear quite close to water.

     A fairly recent, and very unwelcome, addition to Southwestern rivers has been the salt-cedar, or tamarisk. It was planted in peoples' yards as an ornamental, but escaped the cities and became well-established by the rivers, suffocating both willows and cottonwood, and often growing right up to the edge of the rivers. Tamarisk is a tall, unpleasant-looking plant that grows in thickets so dense one can't walk through them. Birds don't nest in them, and they soak up water from the ground, drying up wetlands as quickly as they can establish themselves. Another common Southwest riparian invader is the Russian olive, also a garden escapee. Both plants have the ability to drive out native plants.

     The entire area of the Everglades in Florida is not just a watershed, it's a riparian corridor. This is because the lush vegetation of the Everglades is fed by a vast, shallow sheet of moving water covering the southern end of the state. It is actually a strange kind of stream, in many places no deeper than a foot. Riparian corridors in Florida tend to all follow the same general pattern because the terrain is so level. Cypress trees are found with their roots actually in the water, with cabbage palms growing further away. This riparian corridor once covered nearly all of southern Florida, before it was logged and filled in by people.


Niche is an ecological term that refers to the "place" that an organism fills. Each species fills a number of different niches, that together make its microhabitat and lifestyle. For instance, here are some of the niches an adult trout might fill in a stream:

Predator of drifting insects, minnows, eggs, juvenile fishes, and amphibians. Inhabitant of a deep pool next to a run, with overhead cover.

Species of both plants and animals usually change the niches they occupy as they age. (Ontogenetic changes). For instance, the trout we examined above was at one time a small YOY (young-of-year) or Age I (one year old) trout that filled these niches in the same stream:

Predator of eggs and small creatures, like larvae, on the benthos (bottom of the river). Inhabitant of a run of medium depth, with or without overhead cover.

A species living right next to the young trout, a dace, may nevertheless occupy its own special niches:

Scraper and gatherer of diatoms, algae, protozoa, and bacteria. Inhabitant of a riffle.

Some native trout in the United States

Oncorhynchus clarkii, cutthroat trout: A native trout found only west of the Rocky Mtns., and consisting of 10 subspecies. Most are threatened, some are endangered. Often hybridizes with introduced rainbow trout.

Oncorhynchus mykiss, rainbow or steelhead trout: A trout native only to the Pacific Coast of North America. Now raised in hatcheries and introduced world-wide for sport fishing. An aggressive competitor. The rainbow is the land-locked version of the steelhead trout.

Oncorhynchus gilae, Gila trout: An endangered species living only in the Gila River in Arizona/New Mexico.

Oncorhynchus apache, Apache trout: A threatened species living only in mountain streams in Arizona.

Oncorhynchus aguabonita, golden trout: A threatened trout with several subspecies, one of which is threatened. Native to California.

Salvelinus confluentus, bull trout: An endangered species native to the West.

Salvelinus fontinalis, brook trout: Actually a char. A small, handsome, easy-to-catch trout native to the Great Lakes Region and NE United States, introduced to Western streams.

Salvelinus malma, Dolly Varden: A large char closely related to the bull trout, and native to Pacific Coast states and Northeast Asia.

Salvelinus namaycush, lake trout: Actually a char. Nearly exterminated from the Great Lakes Region by the combined effects of overfishing, introduced fishes, and an intrusion into the great lakes by the sea lamprey, which was caused by human engineering.

Management of Endangered Species

Piscicides (chemicals that kill fish) like Antimycin A are often used to kill off invasive species in a river. The river is then seeded with fry (young trout or other fish from a hatchery) of the endangered, indigenous species. Rivers are then monitored over the long-term, to ensure the survival of the native fish.

Recreation is changing as fishers learn to enjoy fishing for native trout and large minnows, which are often great sport. Fly-fishing is becoming more popular, as people learn about catch-and-release fishing.

Water release methods from dams are changing as flows are timed better to accomodate biosystems, and as water is released from the warmer surfaces of reservoirs, rather than the colder bottom layers. Some dams, having outlived their usefulness, are being removed.

Riparian corridors are being protected from further damage, and rehabilitated by volunteers.

Monitoring tells scientists what is happening with rivers and what changes need to be made.


Genetic diversity refers to the degree of diversity you would see if you were to extract DNA from a collection of animals within one species in one location. It would be analogous to the diversity you would find if you looked at all the humans in New York City. There would be people with all different kinds of genetically-determined traits in your sample. Genetic diversity is essential to the survival of each species. In life, all species are subjected to various deadly events, and streams are no exception. Drought, a particularly cold winter, logging, forest fire, volcanic eruptions, parasites, pathogenic fungi and bacterial infections, a sudden increase in predators or the loss of predators--all these and many more can devastate populations. Genetic diversity ensures that the outcome is a recovery of the species and not permanent extinction. In times of disaster, sometimes only a few members of a group will survive due to a lucky mutation of their genes--and that will be enough to ensure continuation of the species.

Genetic diversity can be (and has been) destroyed when fishes are raised in hatcheries. In truth, only natural spawning processes can ensure genetic diversity. Hatcheries draw from a limited gene pool--a few males and females--to produce enormous numbers of fry, which are then released back into the wild. This process, while meant to ensure strong populations of the fish, has actually propelled many of the species even faster down the road to extinction. What is happening is often called "dilution of the wild gene pool." Streams contain increasingly more hatchery fish and descendants of hatchery fish, and this has compromised their genetic diversity. They are therefore much more likely to succumb to environmental stressors.

Biological diversity refers to the sheer number of species of plants and animals found in one location. Biological diversity is important because it keeps foodwebs from collapsing. Should one species experience a drop in populations, leave the habitat temporarily, or even go extinct, other species are available to predators. Since every link in a biological system depends on other species in the foodweb, biological diversity becomes critical during times of environmental stress. A human analogy can be found in stock portfolios. The person who has invested in the most diverse areas will be the most capable of weathering economic downturns.

Biological diversity, like genetic diversity, is the product of many millions of years of evolution. It cannot be replaced. It is decreased by each species extinction. The only way to protect it--and prevent the collapse of ecological systems--is by preventing mere human carelessness from causing species extinctions.

Habitat diversity is intimately linked with genetic and biological diversity. Each species has its own survival and reproductive strategies, and each depends on special habitats to carry them out. Habitats are created by the processes of natural streamflow, as well as by the rocks, boulders, and trees that become a part of stream channels. Habitats are quickly removed or destroyed by straightening rivers and trying to control them by lining banks with cement and riprap, operation of dams, removal of riparian vegetation, development, and even by apparently well-meaning people who "clean out" stream channels.

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