Connecticut Water Trails Program

Dams

What Is A Dam ?

A dam is a barrier built across a water course to hold back or control the water flow. Some dams divert the flow of river water into a pipeline, canal, or channel. Others raise the level of inland waterways to make them navigable by ships and barges. Many dams harness the energy of falling water to generate electric power. Dams also hold water for drinking and crop irrigation, and provide flood control.

A beaver dam is an example of a small dam. It is made by using sticks and mud to slow down the flow of a stream or a river. This causes water to pool behind the jam of sticks and mud which results in a new pond being built.

Large dams are more complex to build and take a lot of work, power, time and money. A dam can be made of concrete, rocks, wood or earth. An example of a large dam is the Glen Canyon Dam in Arizona. It is about 700 feet tall and is made of concrete.

The most important load that a dam must support is the water behind it. The water pushes on the dam, creating water pressure. Water pressure increases with the depth of the water.

In greater depths, there is more water "piled up," which causes the pressure to be greater at the bottom than at the surface. A dam's design must enable it to withstand greater pressure at the bottom than at the top. As a result, many dams are built in a triangular shape. The wide bottom withstands the great load of the water deep below the surface, while the thinner top of the dam need not use unnecessarily costly materials.

Why Dam?

Dams are built for many reasons:

To control floods.

To make lakes for recreation.

To store water for drinking, industry or irrigation

To raise river levels for navigation (consider the dam-studded Mississippi and Missouri Rivers, which are essentially barge canals at this point).

To store water for hydroelectric power (in earlier times, to power grain- or saw-mills.)

What Do Dams Do?

Whatever their purpose, dams regulate water flow. And that can cause a series of upstream and downstream problems.

Upstream and downstream:

Constant water level prevents sediment from consolidating, turning banks mucky.

Dissolved oxygen may fall, harming fish and shellfish.

Water may warm, injuring or killing fish that need cold water.

Upstream of the dam:

Wildlife habitat is flooded.

Fish are blocked from migrating and spawning (although the decline of the salmon is blamed largely on dams in the Northwest, steelhead, striped bass, sturgeon and alewife have similar spawning habits).

Silt in the dam impoundment can damage or destroy fish spawning grounds.

Downstream of the dam:

By regulating river flow, dams destroy habitat for organisms adapted to rising and falling water.

River stay inside their banks, so floodplains no longer get deposits of fertile sediment .

Dams block sediment flow, causing many changes downstream:

The river bottom becomes more rocky, as sediment no longer fills gaps between larger stones.

Beaches on the nearby ocean are starved of sand normally carried in the river, a particular problem in Southern California.

The same starvation can affect wetlands. The many dams on the Mississippi-Missouri rivers, for example, have deprived wetlands in the Gulf of Mexico of sediment, exposing coastal Louisiana to devastating hurricanes.

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