What is "high water mark"

Tidemark \Tide"mark\, n.

1. a mark on a shore line indicating the normal level of the water at high tide, caused by the action of the flowing water; sometimes also used for the mark left at the point of the normal low tide; -- called also high water mark.

2. [fig.] the maximum level which some activity has reached and from which it has declined; -- called also high water mark.

A high water mark is a point that represents the maximum rise of a body of water over land. Such a mark is often the result of a flood, but high water marks may reflect an all-time high, an annual high (highest level to which water rose that year) or the high point for some other division of time. Knowledge of the high water mark for an area is useful in managing the development of that area, particularly in making preparations for flood surges. High water marks from floods have been measured for planning purposes since at least as far back as the civilizations of ancient Egypt. It is a common practice to create a physical marker indicating one or more of the highest water marks for an area, usually with a line at the level to which the water rose, and a notation of the date on which this high water mark was set. This may be a free-standing flood level sign or other marker, or it may be affixed to a building or other structure that was standing at the time of the flood that set the mark.

A high water mark is not necessarily an actual physical mark, but it is possible for water rising to a high point to leave a lasting physical impression such as floodwater staining. A landscape marking left by the high water mark of ordinary tidal action may be called a strandline and is typically composed of debris left by high tide. The area at the top of a beach where debris is deposited is an example of this phenomenon. Where there are tides, this line is formed by the highest position of the tide, and moves up and down the beach on a fortnightly cycle. The debris is chiefly composed of rotting seaweed, but can also include a large amount of litter, either from ships at sea or from sewage outflows.

A high water mark is a point that represents the maximum rise of a body of water over land.

High water mark may also refer to:

• High-water mark of the Confederacy, the turning point of the Battle of Gettysburg
  • High Water Mark of the Rebellion Monument, a Gettysburg Battlefield memorial
• High water marks, used to determine hedge fund performance fees (see Hedge fund#Fees paid to hedge funds)
• High-water mark (computer security), a computer security model wherein a document takes on the highest level of confidentiality allowed to the last person to access it