Zero Water Level

the zero line of a depth gauge at water measuring stations used to determine the mean long-term water level in rivers. Variations in the level above and below the zero line are calculated with an accuracy of up to 1 cm. The zero water level is related to absolute height, as computed from the mean long-term sea level (in the USSR, from the zero line of the Kronstadt depth gauge). For example, the zero water level at the mouth of the Neva River corresponds to the zero line of the Kronstadt depth gauge, whereas the zero water level at the source of the Neva is approximately 5 m higher.