Negative Effects of Dams on Streams

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.

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.

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.

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.

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.

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Full article: The Effects of Dams

A river ecology encompasses the entire river, from beginning to end. When left in the natural state, the river flows sediment and nutrients along it's path, but dams prevent this natural process, doing damage well beyond the traditional scope of thought.

I also believe that any interruption of the natural stream ecology does damage well beyond the scope of what is generally accepted. Rivers and streams transmit sediment and nutrients that support life all along the stream. Dams prevent much of this from reaching further downstream, decreasing the health of the stream. Most dams also have negative impact on agriculture below the dam, especially during construction and the time it takes to fill before water is allowed to be released again. Fish that spawn upstream are also effected, though workarounds like Fish ladders have helped species like salmon circumvent this problem.

The Effects of Dams - Stream Biology and Ecology