Dams are structures built for a variety of uses, including flood protection, power generation,
agriculture, water supply, and recreation. Most dams are typically engineered to withstand a
flood with a computed risk of occurrence, such as a 100-year flood which has a 1% probability of
occurring in any given year. As inflow to the water pooled behind a dam approaches the design
capacity, emergency measures can usually be taken to release excess water. Also, an emergency
spillway is required to be built into dam structures to release excess water. If emergency
measures are insufficient, the dam structure may be overtopped, which is when water passes
over the top of the dam. Overtopping often leads to dam failure and is the primary cause of
earthen dam failure in the United States. Dam failures can also result from a combination of
causes, including:

- Failure of upstream dams on the same waterway,
- Improper maintenance,
- Negligent operation,
- Internal erosion caused by embankment or foundation leakage or piping, root growth, or
rodent/wildlife activity,
- Earthquake/seismic activity,
- Prolonged periods of rainfall and flooding, which result in overtopping,
- Improper design,
- Inadequate spillway capacity resulting in excess overtopping flows, and
- Intentional structural attacks and cyberattacks.

Water released by a failed dam commonly causes a flood that is catastrophic to life and property.
A catastrophic dam failure could challenge local emergency response capabilities and require
evacuations to save lives. Impacts to life safety will depend on the warning time and the
resources available to notify and evacuate the public. Major loss of life could result, as well as
potentially catastrophic effects to roads, bridges, homes, and other infrastructure. Associated
water quality and health concerns could also be issues. Factors that influence the potential
severity of a full or partial dam failure are the amount of water impounded; the density, type,
and value of development and infrastructure located downstream; the characteristics of the
river channel, and the speed of failure.

Dam induced inundation can also occur from non-failure events when dam outflow exceeds the
capacity of the downstream river channel. This can occur when emergency measures are taken
to release water and prevent a dam failure due to heavy rains and/or high inflows. Emergency
releases are typically considered controlled or designed and are done to protect the dam but
can nevertheless result in flooding downstream. Additionally, outlets and spillways may release
water in a different channel and create additional inundation areas.

Dams may be classified according to the type of construction material used, the methods used
in construction, the slope or cross-section of the dam, the way the dam resists the forces of the
water pressure behind it, the means used for controlling seepage, storage characteristics (on a
watercourse, off-stream, above or below ground level), and occasionally, according to the
purpose of the dam. The materials used for construction of dams include earth, rock, tailings
from mining or milling, concrete, masonry, and combinations of these materials.