In coastal Maine, human activities may impact the estuarine system by altering 

 the estuarine geology and associated erosion and sedimentation rates. Certain 

 upstream and coastal developments and associated dredging and filling alter 

 estuarine geology. The extent of these activities and their potential 

 impacts are covered in chapter 3, "Human Impacts on the Ecosystem." 



Hydrography 



From the hydrographic standpoint estuaries are areas where fresh water mixes 

 with ocean water. (A discussion of marine hydrography is found on page 4-11 in 

 chapter 4, "The Marine System.") This mixing is influenced by many factors, 

 including freshwater flow, winds, basin topography, and tidal exchange. 

 Consequently, areas defined as estuarine change and are not constrained 

 geographically by their enclosing points of land. In fact, Bigelow (1927) 

 considered the entire Gulf of Maine to be an extended estuary, however, the 

 geographical approach used by the NWl will be used here. 



Sufficient data are not available for hydrographic comparisons of the 

 estuarine systems in coastal Maine. Among the many schemes for characterizing 

 estuaries, the Ketchum (1951) flushing model requires data only on fresh water 

 flow, tidal height, and topography to provide a semiquantitative description 

 of the major features of estuarine hydrography. This model will be used as a 

 basis for comparison with specific data on selected estuaries. 



An estuary is a semienclosed coastal body of water that has a free connection 

 with the open sea and within which sea water is diluted measurably with fresh 

 water derived from land drainage (Cameron and Pritchard 1963). The vertical 

 circulation of the water in estuaries is a result of the interaction of salt 

 water and fresh water, which is determined by local topography, freshwater 

 flow, tidal exchange, and local winds. These factors vary on time scales of 

 hours to years and circulation varies accordingly. They are examined 

 individually below. 



Freshwater flow . The principal factor determining the circulation in 

 estuaries is the inflow of fresh water from rivers. Fresh water is less dense 

 than sea water, and when it flows into an estuary it floats on the surface and 

 is separated from the sea water underneath by a strong density interface that 

 inhibits mixing. It floats above the sea water and across the surface towards 

 the open sea, and in the absence of mixing would flow as an intact layer, as 

 an extension of the river. In the absence of mixing, the circulation would 

 exist as a surface flow underlain by a layer of salt water and as the estuary 

 widened downstream the seaward velocity would decrease (figure 5-9). 



In practice, the boundary between the flowing fresh water and static sea water 

 would have to be frictionless for the above conditions to be realized, and it 

 is not. However, in estuaries where the freshwater flow is the dominant flow, 

 estuarine circulation approximates the above description and such estuaries 

 are termed "salt wedge" or "highly stratified" estuaries. 



Tidal exchange (see also "Tides," page 4-18 in chapter 4, "The Marine 



System"). Tidal exchange is the major factor providing mixing energy to 



estuaries. Few estuaries are large enough to have their own tides, and Maine 



estuaries experience an extension of the oceanic tide. The tide enters an 



stuary as a long period wave that moves progressively up the estuary, so that 



5-18 



