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 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, rain that falls in the Rocky Mountains ends up in the Mississippi River.

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 melt water from a glacier, it will carry glacial flour, which is sediment 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. These heavier objects bounce along the bottom in a process called saltation.

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.

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, and they are no longer able to breathe. Fish in acidic lakes simply suffocate to death.

The narrow area alongside a stream that has its own special vegetation is called the riparian corridor. 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. They 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, visible and microscopic, 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.

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 food chain. 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 the communities. Habitats are removed. Slight changes in a stream's insect populations end up affecting the top of the food web, 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.

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. For instance, central California is filled with 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

When a river is prone to flooding, or to meandering out of control and across property lines or roads, we usually channelize it. We may dry up whole sections of river in order to bulldoze it into a tidy, straight line of water, and protect ourselves from its unruly behavior by lining the new channel with concrete or riprap (small boulders crated by truckloads and dumped along the sides of water channels). Unfortunately, this kind of channelizing leads to loss of habitat, as well as increasing the destructive potential of the river.

When towns grow up on floodplains, the annual flooding of the river and the changing of its course will bother the residents, and they will do what they can channelize it. This is a no-win situation for everyone concerned. The river becomes poor in nutrients and poor in habitat, and finally poor in organisms. This means it will no longer support the kind of fish that people like to see in their rivers, although it may often support fish that are not so welcomed by people, like carp. And 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. The unique physical properties of water give rivers powerful force.

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.

It is very important to realize that apart from completely cementing a river in, there is no way to control this increased energy. It will continue to attempt to break out of its channel. It may succeed during a rainstorm, with potentially devastating consequences.

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.

When Hurricane Katrina hit, the entire world witnessed one of the consequences of channelizing rivers in order to build more houses in floodplains, as New Orleans and many of the surrounding parishes sank beneath floodwaters. Speculators and politicians have made a good business out of draining the swamps and marshes of the Mississippi River's delta in order to build new developments. In order to keep the land dry, they've laced the delta with channels that carry the countless streams of the delta out to sea. River deltas always sink, because the sediment that their rivers have deposited on them (through these little streams) over the millennia is very heavy. The streams, however, keep meandering and depositing more sediment, thus building the deltas up even as they are sinking. It is a balance that keeps deltas from ever sinking below sea-level. Now that the streams of the Mississippi Delta have all been channelized, however, no new sediment gets deposited--and the delta has begun to well and truly sink into the Gulf. Levees were built to protect the city and parishes from storm surge caused by hurricanes, but as everyone can now see, once the levees are breached the result is unmitigated disaster. (An event that had been predicted by scientists and ignored by developers and politicians).

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 (Cyprinus carpio), a fish that is not native to America but has populated many of its streams and lakes, will also silt up rivers as it roots about in the mud looking for food.

After land is logged (unless the loggers own the land), people start to move in, filling up the drainage basin with buildings and pavement. This means that there will be more oil, and more lawn fertilizer, herbicides, and pesticides, than ever. It also means pollutants will run straight into streams more than ever. And because each house must have a sewage system to dispose of its inhabitants' wastes, these sewage systems will eventually fail, leaking 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 food chain, chemicals that build up in animal tissues to later poison humans and predators, and even prescription medications like Prozac.

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. They have conceived the bright idea of building pipes beneath major rivers like the Rio Grande to remove water from beneath their riverbeds. Interestingly, a great many people seem to believe that this water is separate from the river water and that it will not draw down the rivers any further than they already are. This is wrong. It is no different from removing water from the rivers themselves. The politicians who, in order to support increased development, have pushed for this measure are likely not so naive. Rather, they simply recognize that drawing water from beneath a river is thus far a good 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.

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.

Perhaps the greatest effect that we humans have had on our rivers and streams is the introduction of fish--and sometimes other organisms--that don't belong there (invasive species). Although some of this has been accidental, like the appearance of animals from peoples' bait buckets or aquariums, or the unfortunate appearance of the zebra mussel in Midwest rivers, most 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. Predators that are introduced to a river will sometimes wipe out a native fish by eating it: they may 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.

Invasive Species

Connecticut Invasive Species

The Future

Connecticut Rivers