The water level reached by a river during a flood; a high water mark.

the upper surface of the underground zone of saturation

The water level in a river or lake at which flood prevention personnel must begin to be on alert and prepare for flood defense.

A chart or map showing water levels; a hydrograph

The level below which the ground is saturated with water; underground water level

The lowest water level in a reservoir at which water can still flow out by gravity through the intake; water below this level cannot be released without a pump.