The Ecology of Dams
Rivers possess a delicate ecology that depends on a regular cycle of disturbance within certain tolerances. The plant and animal communities that inhabit the river and river margins have evolved to adapt to their river's own peculiar pattern of flood and drought, slow and fast current. Dams disrupt this ecology.
There are several types of dams. Check dams prevent flooding of small areas. Diversion dams divert river water to irrigate crops. Large dams may be built for flood control or electrical generation, or both. Flood control dams are often earth dams--made of huge mounds of clay, sand, gravel, and rock--but often made of concrete. Hydroelectric dams are concrete marvels of engineering. This section will examine mostly the large dams: flood control and hydroelectric dams.
Interrupting Natural Cycles
The first effect of a dam is to alter the pattern of disturbances that the plants and animals of a river have evolved for. Many aquatic animals coordinate their reproductive cycles with annual flood seasons. Every flood is valuable in that it takes nutrients from the land and deposits them in the river, providing food for the stream's residents. Floods also provide shallow backwater areas on vegetated and shaded riversides; the young of many animals depend on these backwaters to protect them from large predators.
As an example, a fish on a certain river may only reproduce during April of every year so that its offspring will have abundant food and places to hide. If the flood never comes because a dam holds the river back (because people want the water for themselves), the offspring may be produced during a time when they cannot possibly survive. If the fish can wait until the next flood, which may be in July or may be in October, its young will be born during the wrong time of year, and will have to contend with the absence of their normal food supply and temperatures for which they are not prepared.
Vegetation, too, depends upon these regular cycles of flood. Quite often, people will decide that they can spare no water at all and no flooding will occur. Or they may have built the dams specifically to stop flooding, so they can build houses in the floodplains. When this happens, riparian vegetation, the vegetation bordering the river, changes forever. An example of this may be found in much of the Southwest United States, where enormous floodplains of cottonwood and marsh have been replaced by dry, barren areas of tamarisk and grass.
Armoring the Riverbed
If the dam is allowed to release water from its reservoir, it will often do so only once in awhile, rather than in frequent, small floods as are seen in nature. This leads to scouring and armoring of the riverbed. The higher energy of the sudden floods picks up and removes smaller sediments like silt, sand, and gravel, as well as aquatic plants and animals, leafy debris, and large woody debris. Complex sets of habitats are erased. The riverbed below the dam becomes like a pavement of cobbles and loses its value as habitat for plants, macroinvertebrates, and fish.
Removing Sediment
Another reason that riverbeds become scoured and armored is that dams remove all the sediment from the river. It is natural that the river, which is accustomed to carrying sediment and now has none, will pick up the sediment from the streambed below the dam. It is almost as though the river has been "starved" of its sediment. As in everything else in nature, balance will be achieved one way or the other, often at the expense of one or more species.
What happens to the sediment in a dammed river? It reaches the slow-moving reservoir above the dam and drops out, settling behind the dam. If this seems worrisome to you, it should. Dams are engineered to withstand the force of a certain number of tons of water--however large the reservoir is planned to be. They are not engineered to withstand the additional force of tons of wet sediment pressing on their backsides. The muddier the river, the faster this heap of sediment will build up. What happens when it builds up too high? Either the dam bursts, killing people and destroying settlements downstream, or the reservoir's water pours over the top of the dam. In effect, a huge man-made waterfall has been constructed, and will remain there for thousands of years.
Can we not remove the sediment from behind dams? Unfortunately, the answer is "No--not yet, we can't." There appears to be no safe and economic way to do it.
What about the river downstream of the dam? Isn't its nice clear, cold water a great improvement--especially in regions like the American Southwest where rivers tend to be muddy? The answer is only "Yes" if you have decided the original ecology of the river doesn't matter. If you only want water to look at and drink, then you will be happy enough. If you want a living river ecosystem filled with fish and birds, you will be sadly disappointed. This cold, clear water will be starved of nutrients and provide little or no habitat for animals. In addition, animals that once used the "muddiness" of the river's water to conceal them from predators are now overly vulnerable to predation, and may quickly go extinct. A river with dams eventually becomes little more than a dead channel of water.
Starving the River
Dams hold back not only sediment, but also debris. The life of organisms (including fish) downstream depends on the constant feeding of the river with debris. This debris includes leaves, twigs, branches, and whole trees, as well as the organic remains of dead animals. Debris not only provides food, it provides hiding places for all sizes of animals and surfaces for phytoplankton and microorganisms to grow. Without flooding and without a healthy riparian zone, this debris will be scarce. Adding to the problem, although debris might come from the river above the dam, it is instead trapped in the reservoir, and never appears downstream. The bottom level of the food web is removed. All in all, the loss of sediment and debris means the loss of both nutrients and habitat for most animals.
Changing Temperatures
Temperature is another problem. Rivers tend to be fairly homogenous in temperature. Reservoirs, on the other hand, are layered. They are warm at the top and cold at the bottom. If water is released downstream, it is usually released from the bottom of the dam, which means the water in the river is now colder than it should be. Many macroinvertebrates depend on a regular cycle of temperatures throughout the year. When we change that, we compromise their survival. For instance, a certain stonefly may feel the cold temperatures and delay its metamorphosis. This may mean that at a certain lifestage it will be living in the depth of winter rather than in autumn as it should have been.
Stopping Fish Migration
Fish passage is a concern with dams. Many fishes must move upstream and downstream to complete their lifecycles. Dams are often built without fish ladders. When fish ladders are provided, they seldom work as needed. If enough adult fishes do manage to climb above a dam, there remains the issue of their young: how will they get back downstream? Many are killed by predators while they wander, lost, in the reservoir above the dam. Many are killed in their fall downward through the dam to the river below. They aren't killed by the fall itself, but by the high levels of nitrogen gas at the base of the dam. In other words, like divers who go too deep, they get the "bends."
There are many fishes that cannot climb dam ladders or leap over low dams. Some of these fishes swim upstream every year to breed, then let the water carry them back downstream. The eggs of pelagic spawners float downstream, too, which is why the adults must swim far upriver to breed. Otherwise, the baby fish would soon end up out to sea!
Erosion
Perhaps deadliest of all to salmon and steelhead species is the typical hydropower practice of releasing large amounts of water in powerful surges during the day in order to provide electricity when demand and prices are highest, and cutting down flow during the night in order to replenish reservoirs for the next day. The cyclic floods caused by this popular practice contribute to the extinction of salmon by flushing away their spawning gravels during the day and leaving them high and dry at night. Riverbeds become scoured, stripped of their organic materials, sediment, vegetation, and macroinvertebrates.
Why We Always Seem to Need More Dams
Government institutions such as the United States's Army Corps of Engineers and Bureau of Reclamation campaign mightily for each new dam. They do this because they have no other reason for existing. Their survival depends on building and maintaining dams, as well as other water-conserving projects. Politicians often want to make certain of their constituents wealthy by encouraging "growth," (construction of new buildings and roads) and will join in pressuring for new dams to be built.
But why do we keep needing more of them? This is where the irony lies. Once a dam is built and its reservoir is formed, the region that is served by the dam will be developed. In other words, it will be filled with cities, roads, parking lots, and houses. This, unfortunately, lowers the water table due to water extraction and urban runoff. And that lowers the river even further. Eventually, the new human populace will run out of water--but they will still want to "grow" (sell land for profit). At this point, they will demand yet another dam.
In other words, the building of dams leads to the building of more dams--until there is no water left to take. At this point, the region around the river will be packed with buildings, asphalt, and cement, but the reason people began moving there--often because of the river's beauty--will be gone.
