There are different types of dams classified by the material and design used in construction. These differences influence how and why dams fail. A dam can be classified by its material, indicating whether it is earthen or concrete. Dam components can also include iron, steel or timber or a combination of any of the above.

The different types of dam design include embankment dams, gravity dams, arch dams and buttress dams. Embankment dams are generally made of semi-permeable natural materials such as rock or earth, while the remaining three types rely mainly on concrete.

The function of dams is to block the flow of a river creating a natural storage tank for the now trapped water. A functional dam needs to be strong enough to hold back its reservoir (the Three Gorges dam's massive reservoir is as big as Lake Superior and its normal pool level will reach an elevation of 175 meters) and it needs to be able to pass floodwaters through the structure during heavy rains and periods of increased run-off.[fn]1 Of course, meanwhile it needs to retain enough water to generate electricity (if it is a hydro dam) and
storage capacity if it is a flood control dam.

Sources: Hawker, Patrick, 2000. A Review of the Role of Dams and Flood Management, Thematic Review IV.4: An
Assessment of Flood Control and Management Options, www.dams.org/
“Notable Dam Failures,” Washington State Department of Ecology, 2007.
www.ecy.wa.gov/PROGRAMS/wr/dams/failure.html
“Why Dams Fail?” Federal Emergency Management Agency, 2006. www.fema.gov/hazard/damfailure/why.shtm
“Operations, Monitoring and Decommissioning of Dams Final Version,” World Commission on Dams, November
2000, www.dams.org[/fn]

To do this, dams have built in safety features called spillways, a channel or passageway that allows a controlled release of excess water, used to maintain safe water levels or to slowly empty the reservoir if the dam is cracked or failing.

If the spillways become blocked with debris, like silt, mud or trees, or if the spillway gates (the mechanisms controlling the spillway) malfunction and water cannot be released,
there is a danger that the water level in the reservoir will rise higher than the crest of the dam and spill over. This is known as "overtopping."

A dam's foundation is crucial to the dam’s stability. Just as with a house, a shifting foundation can cause cracking and failure. During an earthquake, for example, a dam undergoes the same stresses as any other structure. Cracks in a dam may indicate structural damage that can lead to collapse.

The US-based Federal Emergency Management Agency (FEMA) defines dam failure as a: "catastrophic type of failure characterized by the sudden, rapid, and uncontrolled
release of impounded water or the likelihood of such an uncontrolled release.