A flood is an overflow of an expanse of water that submerges land. The EU Floods directive
Floods directive
DIRECTIVE 2007/60/EC of the European Parliament and of the Council of 23 October 2007 on the assessment and management of flood risks is commonly known as the ‘Floods Directive’....

 defines a flood as a temporary covering by water of land not normally covered by water. In the sense of "flowing water", the word may also be applied to the inflow of the tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

Flooding may result from the volume of water within a body of water, such as a river
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 or lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

, which overflows or breaks levees, with the result that some of the water escapes its usual boundaries.

While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, it is not a significant flood unless such escapes of water endanger land areas used by man like a village, city or other inhabited area.

Floods can also occur in rivers, when flow exceeds the capacity of the river channel, particularly at bends or meanders.
Unanswered Questions

1874    A flood on the Mill River in Massachusetts destroys much of four villages and kills 139 people.

1970    In Vietnam, the worst monsoon to hit the area in six years causes large floods, kills 293, leaves 200,000 homeless and virtually halts the Vietnam War.

1996    Storms provoke severe flooding on the Saguenay River, beginning one of Quebec's costliest natural disasters ever.

2004    Hurricane Jeanne: At least 1,070 in Haiti are reported to have been killed by floods.