Highlights: introduction | issue identification | dam and river ecosystem basics | legal aspects | economic considerations | decision-making process | conclusions

Dam and river ecosystem basics

A river is a stream of water that flows over the land surface. Rivers naturally flow from high to low elevations under the force of gravity. After a dam is built across a river, the water backs up behind the dam, flooding the land to the elevation of the dam. This forms a lake or pond, commonly referred to as an impoundment. Most dams in Wisconsin are “run-of-the-river” dams, meaning that the amount of water flowing through the dam is the same amount flowing into the impoundment from the river upstream. Run-of-the-river dams have no way to control the level of the impoundment and do not provide flood control.

A river gathers most of its water from rain and snowmelt in its watershed, the area of land that captures water and directs it to the river. This water either flows over the land surface or seeps underground as groundwater to the river. For most rivers in Wisconsin, the water table is above the level of the river, and so groundwater flows into the river (high to low; fig. 1).
 

Figure 1. The groundwater that flows into a stream is called the stream’s baseflow.
 

A dam raises the water level of the impoundment above the natural water level in the river and may cause water to flow out of the impoundment into the adjacent groundwater aquifer. This in turn raises the level of the water table in the vicinity of the impoundment. Should the dam be drawn down back to the original level of the river, the water table will drop back to its original level, barring any other hydrologic changes in the watershed (fig. 2).
 

Figure 2.  An artificial impoundment creates a groundwater mound near the impoundment.
 

Wetlands can be found adjacent to rivers. These areas are important ecologically for maintaining water quality, providing habitat, and protecting plant and animal species diversity. When a river is dammed, the wetlands next to the river will be flooded behind the dam; however, new wetlands may form next to the impoundment. Just as the water table will return to its original configuration if the dam is drawn down and removed, wetlands can be expected to restore themselves in their original places.

Not only do dams impede the flow of water in a river system, they also block the flow of sediment (silt, sand, and so forth) in the river. Rivers naturally erode, carry, and deposit sediment. These processes are what shape the river, form meanders, pools, and riffles. The river deposits its sediment load in the impoundment when the flow velocity slows and the particles settle out. Over time, sediment can fill in the impoundment. Eventually, the impoundment may become so shallow that the sediment must be removed by dredging or other means. The river downstream of the dam is “starved” for sediment because the sediment naturally flowing in the river has been trapped behind the dam. The water flowing through the outlet of the dam may be relatively clear, and carry little sediment.

Water flowing over a dam can cause scour holes to form immediately below the dam. Scour holes may undercut the foundation of the dam threatening the integrity of the structure. In addition, the currents in scour holes present a hazard to swimmers (fig. 3).
 

Figure 3. High-energy flow of water over the former Rockdale Dam erodes a scour hole.  (Photo courtesy of Tom Hooyer, Wisconsin Geological and Natural History Survey.)

The quality of water in the river, free flowing or impounded, greatly depends on the types of human activities within the watershed. The chemical composition of water and the relationship of its chemistry to its intended uses, such as support of aquatic life, drinking water, and recreation, determine water quality. Water-quality factors include temperature, dissolved oxygen, dissolved minerals, nutrients, organic chemicals (including pesticides), pathogens, and sediment. Dams not only impound the water of a river, but also impound the dissolved and suspended substances carried in the water. Free-flowing rivers are constantly flushing themselves; impoundments tend to trap these substances and cause water-quality problems. Algae blooms may develop as a result of high nutrient concentrations deposited with the sediment. These reduce the amount of dissolved oxygen in the water and the amount of penetrating sunlight, which can have an impact on other biota in the water and the aesthetic appeal of the water in the impoundment.

Building a dam on a river has major implications for the biota found in the river system. Because fish and other biota cannot move past a dam, the dam effectively splits the river into separate ecological zones: the river above the dam and the river below the dam. Fish passages may be added to a dam to help fish move up and downstream, but they are not always effective. Although the free movement of fish can sustain a healthy fishery, a dam may be a barrier to the movement of unwanted invasive species. Water tends to warm more in an impoundment than in a free-flowing river, which may affect the types of fish found upstream, in, and downstream of the impoundment. Impounded and free-flowing river systems provide habitat for amphibians, reptiles, birds and mammals.

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