Thursday, February 18, 2010

Seasonal Water Levels In The Marsh

There is a seasonal change in the average water level of Beaver Creek Marsh.  In the months between April and October, the water is lower in the marsh, while between November and March, the water is higher.  The seasonal average water level changes approximately 2 feet.  As water level changes, water moves into and out of the margins of the marsh and large areas are flooded or dried.  Zonation of plant species in the marsh depends in part upon this seasonal flooding and the relative tolerance of species to flooding.  

Monthly mean tidal heights (meters) relative to Mean Sea Level

Looking at the monthly mean tidal height in South Beach, which is 7 miles to the north, we can see the same seasonal cycle of average water levels that is evident in the marsh.  In November - March, sea level is above annual mean sea level (MSL).  In April - October, sea level is below MSL.  The change in monthly mean sea level over an annual cycle is approximately 1 foot.  While monthly mean sea level appears to control water level in the marsh, it does not explain the magnitude of change.  Monthly mean rainfall in South Beach follows the same pattern shown by tidal height, with higher rainfall in fall and winter months and lower rainfall in spring and summer months.  The additional rainfall input to the Beaver Creek watershed during the fall and winter probably accounts for the greater magnitude of water level rise in the marsh, beyond that associated with tide level.  

Monthly mean precipitation (inches) in South Beach, OR

This description is oversimplified since ocean beach sand levels and debris dams can contribute to varying water levels in the marsh, but their effects generally conform to the same seasonal cycle of higher and lower average water level.  Superimposed on the average seasonal cycle are the additional effects of daily cycles of tides, wave height, wind, and rainfall.  During the rainy season, these factors can push water back into the creek and marsh as rainfall puts water into the watershed.  During the dry season, effects of weather are minimal and slow draining of the water table generally lowers marsh levels.

Daily tidal height (feet) in South Beach, OR

Posted by michael     

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