The mean high water spring is the highest level to which spring tides reach on the average over a period of time (often 19 years). This level is generally known as "high water mark". Note: In Queensland, MHWS is specifically defined by legislation. Refer to http://www.msq.qld.gov.au/qt/msq.nsf/index/tides_notes. MHWS is lower than HAT, see http://www.msq.qld.gov.au/qt/msq.nsf/index/tides_tidal_planes for further information.