Introduction to The Cost of Taming a River
In summer 1993, unusually heavy rainfall turned the Mississippi River into a powerful, sprawling force. As the river level rose higher and higher, riverside communities scrambled to maintain their levees, wide walls of earth built to contain the river when it rises to flood stage. But by August, the Mississippi had broken through nearly 70 percent of the more than 1,500 levees, carving soil from the land and sweeping across forests and farmlands. In all, 93,000 square kilometers (36,000 square miles) of land were submerged. The damage was immense, with 50 people killed, tens of thousands left homeless, and billions of dollars in property destroyed.
In late 1993, Europeans also suffered from rivers on the rampage. Flooding of the Niers River in the Netherlands and the Aisne River in France forced thousands of people to abandon their homes for higher ground. The Rhine River, which flows through France, Germany, the Netherlands, and Switzerland, overflowed its banks in what was called that river's worst flooding in 100 years. In the German city of Cologne, for example, the floodwaters drove an estimated 50,000 people from their homes.
Afterward, Americans and Europeans alike considered the role humanity may have played in the tragedies. Community leaders in the United States in particular questioned the wisdom of founding towns and cities so close to riverbanks. Some experts claimed that levees and other “river improvement” projects had actually increased the damage caused by major floods.
Such ideas have been known to scientists since the 1970's. Before that time, research was very sparse on whether river management had unintended consequences. But since the mid-1970's, studies have shown that rivers are varied and changeable and that attempting to control them may cause unexpected problems—not just for people living in the area, but also for the animals and plants that make the river their home. That research has prompted new ideas about how rivers should be managed and has encouraged attempts to restore selected rivers to a more natural state.
River Settlement and Civilization
Rivers have long been important to humanity, because they can provide us with water, food, and transportation. For these reasons, human history is in many ways a history of rivers. The great river valleys of Asia, Egypt, and the Middle East, where civilizations first arose, show evidence of thousands of years of human settlement. People in early civilizations must have quickly discovered that the banks of rivers were ideal places to live. There, the soil is fertile and the ready supply of water can be used for agriculture as well as for drinking.
Eventually, people learned to modify rivers to suit their needs–for instance, by damming a river to create a reservoir for use during dry seasons, when less water flowed through the channel. By draining the marshy land alongside rivers, communities created enough dry land to accommodate towns and farms. By constructing levees, they were able to keep the rivers in their channels, at least during mild floods.
In the United States, such projects are older than the nation itself. When periodic floods interrupted the early settlement of a flood-prone area, authorities responded by building levees, dams, or other structures to control the river. Those measures kept the river in its channel and attracted more settlers to the region. As towns grew up, the flood-control structures became more elaborate. The first levee along the Mississippi River was constructed in 1717. By 1858, the river was lined with more than 3,200 kilometers (2,000 miles) of levees. Since then, municipal, state, and federal agencies in the United States have spent hundreds of billions of dollars altering the natural flow of rivers and modifying river channels.
The Nature of A Natural River
Research into the nature of rivers has helped scientists understand how these changes affect river ecosystems (communities of plants, animals, and microbes along with the nonliving features of their environment). On the simplest level, rivers are products of gravity, which forces water to run downhill, following the shortest available route. Beginning as tiny rivulets on a slope, water creates narrow channels that join to form streams and then rivers. Stream ecosystems spread out like ribbons across the landscape, continually merging to form increasingly larger rivers as they flow down to a lake or the sea.
River ecosystems differ widely—according to the type of landscape they pass through, the climate, and a host of other factors. Even a single river is not the same ecosystem throughout its length. A small stream may support less than 10 fish species, for example, while the larger river it becomes downstream may contain 50 to 100 species.
Nevertheless, river ecosystems share several characteristics. All rivers erode (wear away) their channels and carry the eroded material downstream. All have natural cycles of high flow, normal flow, and low flow, depending upon the amount of rainfall and ground water that enters the river. Every natural river overflows its banks at one time or another, though only some rivers flood regularly—typically in the spring, when rainfall is heavy and snow is melting.
Life In A River Ecosystem
Animal life in almost all natural streams and rivers is richly varied. A single ecosystem is likely to contain many different species of fish, some of which may be important to people as sources of food or for sport fishing. Freshwater clams inhabit the river bottom, and snails cling to plants rooted in the streambed. Amphibians, such as salamanders and frogs, are equally at home in the water and on nearby land, as are certain reptiles such as water-loving snakes and turtles.
Many species of insects abound in and near rivers. Some species hover over the water, some float on the surface, others swim through it, and still others crawl along or burrow under sediment on the bottom. Insects are a major source of food for larger aquatic animals and even for other insects, such as the diving beetle and common water strider.
Aquatic animals also feed on plants growing in the water. The most common and widespread types are microscopic algae, which may consist of single cells, multicellular filaments, or colonies. Often, these films of algae are barely visible to the naked eye. Mosses and liverworts are somewhat higher forms of plants found in most streams and rivers. Larger, rooted plants common to many river ecosystems include water lilies, cattails, and water milfoil. The array of species of plants provides food and shelter for a variety of animals.
Alternating patterns of deep and shallow water along the length of the river also give animals and plants varied habitat. Areas of deep water are called pools, and places where the water is shallow are called riffles. Fish seek out the tranquil waters in pools during times of high flow, when currents are swift and turbulent. Because water in a pool is deeper than in other parts of the channel, pools enable some aquatic animals to survive when water levels drop dangerously. Water over a riffle, on the other hand, is relatively shallow, and currents there are more rapid and turbulent. Certain plant and insect species have adapted to life in this fast lane. The larvae of caddis flies, for example, spin cone-shaped nets to capture food particles swept along in the swift currents. The disklike body of a water penny (the larva of a riffle beetle) forms a suction so strong that it can cling to rocks in the most rapid rivers.
Shore and River Intertwined
More than any other ecosystem, rivers are characterized by their edges. Plants all along the riverbank affect life in the stream. Needles, leaves, and twigs that fall into the water provide food for insects, snails, worms, and fish. Dead trees that fall into rivers can change the shape of the channel or form dams that force the water to seek a new path.
The shade cast across the water by plants on land limits the types of species found in the river. A small stream passing through a dense forest may receive so little sun that only aquatic plants that do not require direct sunlight, such as mosses and liverworts, can survive. For wider streams and rivers, land plants cast shade primarily along the river edges. Sunlight streaming into the center of the channel encourages the growth of algae.
Even the plants on land well away from the riverbanks can be an important part of a river ecosystem. Ground water flowing toward the river channel passes below trees and other plants, and their roots take up more than half the nutrients, such as nitrogen and phosphorus, in the water. This filtering greatly improves the purity of the river water.
Flood Plains
Life on land and in the river are perhaps most closely linked in flood plains—wide, flat regions that extend on either side of a river. Rivers form flood plains over thousands of years. The process begins where a river flows into a low valley and a single channel may braid into multiple channels. During a flood, the multiple channels overflow their banks. As the water spreads out across the land, it slows, and any soil or gravel being carried along drops out as sediment. Over time, the build-up of sediment levels the landscape, creating the extensive flat surface of a flood plain. In just this way, the Brahmaputra, Ganges, and Meghna rivers, which flood every rainy season, have created the broad plains that nearly cover what is now Bangladesh in south Asia.
Many flood plains are underwater at least once every year. There, the only plants that thrive may be grasses or shrubs. But some flood plains are rarely flooded at intervals of less than hundreds of years. Flooding that infrequent permits the growth of forests that cover large areas and contain large, old trees.
The Importance of Flooding
Floods are important natural processes that do more than create flood plains, however. Most stream channels were first carved out during floods, for example, and the turbulence and swift currents of a flood scour out pools and drop sediment to create riffles.
Where flooding occurs with seasonal regularity, aquatic life may require floods to thrive. Many fish species reproduce toward the end of flood seasons, when submerged land plants provide a plentiful source of food for hatchlings. Even in areas where flooding occurs more rarely, fish seem to benefit from floodwaters. River ecologists reported in 1989 that more fish appear in major world rivers in years following floods than in years with relatively constant low flows, particularly in tropical rivers.
Plants growing alongside the river also benefit from flooding. River sediments commonly contain organic material rich in nutrients essential for plant growth, and when floodwaters deposit this sediment, they enrich the soil. After the 1993 Mississippi flood, for example, biologists noted an explosion in the growth of grasses and other plants alongside the river.
Major floods that extend far beyond the banks of the normal stream channel can benefit ecosystems for miles on either side of a river. The Mississippi flood carried fish to ponds and streams where they had not previously been seen. Researchers found increases not only in the numbers of fish in those areas but in the populations of eagles and hawks, which eat fish.
Altering Rivers to Prevent Floods
Flood cycles and the other dynamic characteristics of rivers make these ecosystems among the most complex on Earth. Unfortunately, that changeability almost always causes problems for people living near the water's edge.
Most river management projects—all of which have drawbacks as well as benefits—try to avoid those problems by placing rivers under human control. One aim is to hold back river waters in order to prevent, or at least reduce, the damage caused by floods. Another is to modify the river channel so that the land drains more rapidly after floods or during storms. A third goal is to capture and store water to ensure a reliable supply the year around.
Levees (dikes)
A common method of holding back river waters is to construct levees, which are also called dikes. These structures are usually made of earth piled higher than the water level would rise during a typical flood. Another way of constraining floodwater is by covering the riverbank with concrete or another material to prevent the river from eroding its banks. Such structures are called revetments or armored banks.
Levees, dikes, and revetments all constrict the flow of water to a central channel. That keeps floodwaters out of property flanking the river, but it causes other problems because it increases the speed of the current. The effect is similar to running the water through a narrow pipe instead of letting it flow over a broad stretch of land. The force of water at the end of the pipe will be much greater than it is at any point along the wide sheet of water. The increased power of the water in a river altered in this way causes the riverbank downstream to erode more rapidly than before the levees were built. Downstream communities may end up requiring levees along their waterfronts to combat erosion caused by levees upstream.
The system of levees on the Mississippi increased the damage to downstream communities during the 1993 floods. The levees forced water downstream with unnatural speed and power, so that when high waters broke through downstream levees, waves flowed farther across the flood plain than ever before. Then, when the waters began to recede, the levees prevented the river's immediate return to the channel, making flood conditions last longer than they would have if the river had been unaltered.
Levees also cause trouble upstream. Water in a natural channel must pick up speed as it nears an altered section, where the water flows more swiftly. The force of the accelerating water causes erosion in the riverbed directly upstream from the altered site. That erosion steadily advances upstream. As it does so, a heavier and heavier load of sediment is gouged from the riverbed and carried downstream. All that sediment eventually makes the river shallower and wider, conditions that tend to increase the summertime water temperature. Many forms of aquatic life cannot tolerate the changes.
This destructive chain of events has occurred in the Mississippi River since levee building began in the 1800's. So that sediment would not deposit at the mouth of the river and make it too shallow for navigation, engineers in the late 1800's built leveelike structures called jetties out into the Gulf of Mexico. Ecologists are now worried that funneling the heavy sediment into the Gulf is affecting fish populations there.
Benefits and Drawbacks of Other River Alterations
Another type of river-alteration project is designed to drain adjacent land during floods or storms. Landowners bury ceramic pipes below the surface in such a way that gravity pulls ground water through the pipes and down to the stream or river. Pumps can hasten the drainage. The system of pipes allows lowlands flanking a river to be planted with crops, often very near the riverbank. The remaining fringe of streamside habitat is generally too small to support enough plants and insects to sustain many species of amphibians and reptiles, however, and so the river ecosystem is changed. Water that runs off the fields is also likely to contain pesticides and animal wastes, which will directly enter the stream without the benefit of being filtered through plant roots. The pollution can greatly alter river life and even kill off some species.
Any method of forcing a river to flow more quickly can also help drain adjacent land and make it suitable for agriculture or construction. The simplest technique is to remove obstructions in the water. Almost every municipal water-management plan in the United States calls for removing wood from urban streams. Most large rivers in the nation are “snagged” regularly to remove logs and jams. Doing so removes important sources of food for aquatic animals, however. And because snagging increases the speed of water through the channel, it can cause erosion problems similar to those caused by levees.
Straightening A River Bed
Straightening projects are another technique for routing water out of an area quickly. Straightening projects involve digging a channel between two bends. The new channel is made straighter and steeper than the natural river, so that gravity pulls the water into the new channel, leaving the curves, called meanders, to become marshy lakes that may eventually fill in and disappear.
Obviously, as meanders dry out, life there is reduced to creatures that can live in marshy habitats. Straightening projects can also have other, less-predictable consequences. Various studies have shown, for example, that the altered section is likely to contain fewer fish species than lived in the meanders. Among the species that remain, the average weight of the fish generally falls.
Straightening projects usually involve clearing vegetation to construct the new channel, and the loss of plants can also cause ecological problems. More sunlight penetrates the water, and the temperature of the water rises beyond the level tolerated by some aquatic creatures. Amphibians, reptiles, and other small animals lose their streamside hiding places from hungry predators such as otters and minks. Without the sheltering effect of trees and other plants, daily and seasonal variations in temperature become more extreme and also occur more rapidly than in the untouched river. Some forms of aquatic life can no longer exist under these new conditions.
Dredging A Channel
Deepening a river channel by dredging (scraping) out sediment can also speed the routing of water. Some dredging projects break up a riverbed and remove debris. Engineers also dredge to form one steeper channel where there were several braided channels. Sometimes that process is hastened by filling in the unwanted channels.
Dredging can disrupt a river ecosystem in several ways. The procedure removes the river's natural pools and riffles, eliminating the variety of habitat that helps support a range of creatures. Dredging can also remove the aquatic plants, roots and all, that sustain fish. Finally, if dredging removes silt (fine soil particles carried by water) from the riverbed, the environment for plant life changes. As some plants die off, so do the aquatic animals that feed off them.
Probably the most dramatic way people alter river systems is by constructing dams. There are several types of dams, all of which slow or stop the flow of a stream or river. The water backs up behind the dam, forming a reservoir that provides a reliable source of water for irrigating fields and for supplying towns and communities nearby.
Damming A River For Hydroelectric Generation
Some dams are used to generate electricity. Water falling through the dam spins a turbine, which turns a generator, a device that converts mechanical energy into electricity. Many dams are used as a supplement to a community's power supply, rather than as the primary source of electricity. In this case, engineers hold water back during daylight hours and then release it at night, when demand for electricity tends to be greater. Because those fluctuations from low to high flow are more rapid than in natural cycles, they can prove extremely harmful to aquatic organisms downstream. Species of small fish, very young fish, and species of insects normally found in waters along the banks are frequently missing altogether.
Flow-control dams are designed to smooth out variations in the volume of water passing through a river from season to season. Maintaining a consistent flow downstream ensures an adequate supply to sustain aquatic life and permits year-round navigation on the river. The dam stores water during periods of high flow to prevent flooding downstream, and dam authorities release water during periods of lower flow.
Flow-control dams, levees, and other river projects can reduce the effect of small floods—generally ones that recur at intervals of several decades. But no existing river control project is designed to eliminate the effects of floods that are so large they would occur only once in several centuries. In fact, major floods such as those of the 1990's may cause so much damage in some areas that the costs of the flood outweigh the benefits of developing the land in the first place. The very presence of levees, dams, and other flood controls may indirectly increase the damage toll of extremely large floods, because the structures encourage more people to settle on the flood plain than would risk it otherwise.
Balancing People's Needs With Those of the River
It's important to remember that almost every member of U.S. society benefits directly from the changes made to streams and rivers. They help provide drinking water, waste treatment, and food, and they are useful to the production of many manufactured goods. But humanity has also paid a price for altering rivers to suit human needs. The U.S. Congress appropriated $5.7 billion to pay for damages for the 1993 Midwestern floods, for example, and thousands of citizens suffered emotional turmoil beyond any dollar figure.
Scientists have no simple way to resolve the conflict between our need to use rivers and our desire to avoid the problems river alteration projects cause. It is impossible to move New Orleans and St. Louis, Mo., for example, even though those communities were founded perilously close to a river prone to flooding.
We can make a difference, however, in the decisions made today. An increasing number of experts say that programs that provide flood insurance, such as the National Flood Insurance Program, should be revised so that they discourage, rather than encourage, development on flood plains. And ecologists urge government officials to create programs that recognize the dynamic nature of rivers and prompt wise use of their resources.
Similarly, scientists see no quick fixes for river ecosystems that have been extensively altered. Unfortunately, most of the rivers of North America fall into this category, according to a 1982 project conducted by the U.S. National Park Service. The project, the Nationwide Rivers Inventory, sought to identify high-quality, free-flowing streams longer than 200 kilometers (124 miles) that could be protected from development. In the lower 48 states, only 42 rivers—2 percent of the total—met the criteria, according to Arthur Benke, a stream ecologist at the University of Alabama in Tuscaloosa who reviewed the data in 1988.
Restoring A River
On the other hand, it is possible to restore a river, or at least sections of it, to a more natural condition. In one such project underway in 1994, 35 kilometers (22 miles) of Florida's Kissimmee River were being changed from an artificially straight channel to a 69-kilometer (43-mile) reach of meandering river.
One of the most effective restoration approaches may be to avoid “correcting the damage” caused by certain floods. A major flood—one that happens once in 100 years—can obliterate artificial structures and carve out new channels, full of riffles and pools. The floodwaters can move a tremendous volume of sediment, enough to remake the landscape. In 1993, for example, the Mississippi dropped sediment up to 1.8 meters (6 feet) deep in some places. For rivers earmarked for restoration, such a flood could correct more mistakes and restore more ecological function than any government project. Experts thus say the challenge is to develop sound after-the-flood policies that will identify the benefits of a flood—and other natural features of the ecosystem—before we rush in to try to force the river under our control.