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Frequently Asked Questions – Dams

What are some common problems with dams as they age?

The most common problems found during dam inspections are undesirable woody vegetation on the embankment, deteriorated concrete, inoperable gates and corroded outlet pipes.

Many of the dams in Wisconsin were built in the late 1800s and early 1900s. Over time, water pressure and weathering will slowly break down a dam. Dams need constant maintenance and repairs. If dams are allowed to naturally degrade, they have a greater risk for problems such as sudden breaks during flood conditions. The possibility for loss of life and property damage makes dam maintenance an important issue. Dams left to deteriorate in place can also pose a threat to the life and health of the public using waterways for swimming and boating.

What are the environmental impacts of dams?

Dams are usually built to temporarily slow the flow of water, creating an artificial impoundment, or lake, just upstream of the dam. Water held by the dam is often warmer than the natural water temperature and attracts different fish and mammal species. Fish such as carp prefer the warm water of dams, while trout prefer the cooler, flowing water of a river. Dams prevent native fish from utilizing different stream segments that can be important seasonally or during different stages in their life. Naturally flowing rivers are often cleaner and clearer in water quality than water held by a dam, since nutrients and sediment can be naturally discharged in flowing water. Impoundments created by dams sometimes need to be dredged to remove sediment buildup behind the dam. Beavers and muskrats are also attracted to dams. They can burrow into embankments and compromise the integrity of the structure creating public safety concerns.

What benefits are gained by removing dams?

Dam removals are often a necessity caused by the high cost of dam reconstruction and maintenance or the liability of dam ownership. Removing a dam often removes a hazard to swimmers and boaters in the area and those who live downstream. Recent removals of dams in Wisconsin have also shown a decrease in carp populations and increase in trout species in the area of removal, and have shown a higher water quality and biodiversity after removal. For more information on removals, see Dam removal.

What role does a dam play in heavy rainfalls or flooding situations?

In heavy rainfall or flooding, some dams are designed to hold back some of the water flowing downstream, protecting people and property downstream from higher water. This naturally results in higher water upstream of the dam but auxiliary spillways are designed to pass excess floodwaters downstream. However, most dams in the state are not designed to function as flood control structures. They are run-of-the-river dams designed to hold relatively stable water levels in the impoundment. Flood situations can sometimes exceed the capabilities of either type dam, resulting in a failure and a more drastic and sudden flooding of downstream property. Constant maintenance of dams and awareness of dam operators is essential to protecting the public in flood situations.

Why and when are dams open or closed?

A dam operator can usually control the flow of water and the size of impoundment by opening or closing gates. Gates might be opened when water levels are high and the impoundment needs to be lowered, and may be closed if water levels in the impoundment are low. Many dams have authorized levels that they must try to maintain in the impoundment. Dams must also maintain a minimum downstream flow to protect aquatic resources. All dams should have a written plan for when it is appropriate to open or close gates. This plan is known as an inspection, operation and maintenance plan.

I’m interested in buying some property that has a dam on it. What are some of the responsibilities I will be acquiring?

In Wisconsin, prospective dam owners must be able to prove they are financially capable of maintaining a dam before ownership can be transferred. They will also be potentially liable of any damage caused by the misoperation or failure of their dam. Contact your regional DNR water management engineer for more information. You may also check out our owner responsibility fact sheet [PDF] for more information about the legal responsibilities faced by dam owners.


Additional Resources

Dam terminology

Graphical examples of dams and their components

Information on legal responsibilities, erosion and more


Contact information
DNR Dam Safety
DNR Dam Safety Program, WT/3
101 S Webster St
PO Box 7921
Madison WI 53707–7921