MARSH, SALT. A marsh periodically flooded by salt water. 



MASS TRANSPORT. The net transfer of water by wave action in the direction 

 of wave travel. See also ORBIT. 



MEAN DIAMETER, GEOMETRIC. See GEOMETRIC MEAN DIAMETER. 



MEAN HIGH WATER (MHW) . The average height of the high waters over a 19-year 

 period. For shorter periods of observations, corrections are applied to 

 eliminate known variations and reduce the results to the equivalent of a 

 mean 19-year value. All high water heights are included in the average 

 where the type of tide is either semidiurnal or mixed. Only the higher 

 high water heights are included in the average where the type of tide is 

 diurnal. So determined, mean high water in the latter case is the same as 

 mean higher high water. 



MEAN HIGH WATER SPRINGS. The average height of the high waters occurring at 

 the time of spring tide. Frequently abbreviated to HIGH WATER SPRINGS. 



MEAN HIGHER HIGH WATER (MHHW) . The average height of the higher high waters 

 over a 19-year period. For shorter periods of observation, corrections 

 are applied to eliminate known variations and reduce the result to the 

 equivalent of a mean 19-year value. 



MEAN LOW WATER (MLW) . The average height of the low waters over a 19-year 

 period. For shorter periods of observations, corrections are applied to 

 eliminate known variations and reduce the results to the equivalent of a 

 mean 19-year value. All low water heights are included in the average 

 where the type of tide is either semidiurnal or mixed. Only lower low 

 water heights are included in the average where the type of tide is 

 diurnal. So determined, mean low water in the latter case is the same as 

 mean lower low water. 



MEAN LOW WATER SPRINGS. The average height of low waters occurring at the 

 time of the spring tides. It is usually derived by taking a plane 

 depressed below the half-tide level by an amount equal to one-half the 

 spring range of tide, necessary corrections being applied to reduce the 

 result to a mean value. This plane is used to a considerable extent for 

 hydrographic work outside of the United States and is the plane of 

 reference for the Pacific approaches to the Panama Canal. Frequently 

 abbreviated to LOW WATER SPRINGS. 



MEAN LOWER LOW WATER (MLLW) . The average height of the lower low waters over 

 a 19-year period. For shorter periods of observations, corrections are 

 applied to eliminate known variations and reduce the results to the 

 equivalent of a mean 19-year value. Frequently abbreviated to LOWER LOW 

 WATER. 



MEAN SEA LEVEL. The average height of the surface of the sea for all stages 

 of the tide over a 19-year period, usually determined from hourly height 

 readings. Not necessarily equal to MEAN TIDE LEVEL. 



MEAN TIDE LEVEL. A plane midway between MEAN HIGH WATER and MEAN LOW WATER. 

 Not necessarily equal to MEAN SEA LEVEL. Also HALF-TIDE LEVEL. 



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